Definition: A block explorer is a web-based tool that indexes and displays blockchain data including blocks, transactions, addresses, and smart contracts.

How They Work: Explorers run full nodes that sync with the blockchain network. They parse every block, extract transaction data, decode smart contract interactions, and store everything in searchable databases.

Key Insight: The data shown on explorers is the same data stored on every full node - explorers just make it accessible without running your own node.

Transaction Verification: Confirm that your transaction was included in a block and see exactly what happened

Address Tracking: Monitor any wallet's complete transaction history, token balances, and activity

Contract Interaction: Read and write to smart contracts directly, view source code, and understand protocols

Debugging: Diagnose why transactions failed, trace internal calls, and analyze gas usage

Research: Investigate protocols, track team wallets, and verify project claims

Search Bar: Enter transaction hashes, addresses, block numbers, token names, or ENS names

Transaction View: Shows sender, receiver, value, gas, status, and decoded input data

Address View: Displays balance, transaction history, token holdings, and internal transactions

Block View: Lists all transactions in a block, miner/validator, block reward, and gas used

Contract Tab: Source code, ABI, read/write functions, and event logs (for verified contracts)

Transaction Hash: Unique 66-character identifier (0x + 64 hex chars) - share this to reference any transaction

From/To: Sender address and recipient (address or contract)

Value: Native token amount transferred (ETH, BNB, MATIC, etc.)

Gas Price & Gas Used: Fee paid = Gas Used × Gas Price. Higher priority = higher gas price

Input Data: Encoded function call for contract interactions - explorers decode this if the contract is verified

Block Number: Which block included this transaction - determines finality

Success: Transaction executed completely - state changes applied

Failed: Transaction reverted - gas was consumed but state unchanged. Common reasons:
- Insufficient balance or allowance
- Slippage too low (price moved)
- Contract logic rejected the call
- Out of gas

Pending: Transaction submitted but not yet included in a block. May be stuck due to low gas or nonce issues

Dropped: Transaction removed from mempool (usually replaced or expired)

What Are Internal Transactions?: When a smart contract calls another contract or sends ETH, these are internal transactions. They don't have their own transaction hash but are part of the parent transaction.

Why They Matter: A single swap on Uniswap may trigger dozens of internal transactions as tokens move between contracts. Understanding internal transactions reveals what actually happened.

Viewing Internal Transactions: Click the "Internal Transactions" tab on Etherscan. For detailed call traces, use specialized tools like samczsun's transaction tracer.

What Are Event Logs?: Smart contracts emit events to record what happened. Events are indexed for efficient querying and don't affect contract state.

Common Events:
- Transfer(from, to, amount) - ERC20/ERC721 transfers
- Approval(owner, spender, amount) - Token approvals
- Swap(sender, amount0In, amount1In, amount0Out, amount1Out, to) - DEX swaps

Reading Logs: The "Logs" tab shows all events emitted. Topics are indexed parameters (searchable), data contains non-indexed parameters.

EOA (Externally Owned Account):
- Controlled by a private key
- Can initiate transactions
- Has no code associated
- Examples: Your MetaMask wallet, exchange hot wallets

Contract Address:
- Contains bytecode that executes
- Cannot initiate transactions (only respond)
- Created by deploying code
- Examples: Uniswap Router, USDC token contract

How to Tell: On Etherscan, contracts show a "Contract" tab with code. EOAs only show transaction history.

Why Verification Matters: Verified contracts show human-readable source code. Unverified contracts only show raw bytecode - impossible to understand without decompiling.

Verification Process: Developers submit their source code and compiler settings. The explorer compiles it and checks if the bytecode matches what's deployed.

Trust Implications: Verified code can be audited and understood. Unverified contracts are black boxes - interact with extreme caution.

Alternative: Sourcify: Decentralized contract verification that stores source code on IPFS, not dependent on centralized services.

Read Contract: Call view/pure functions that return data without changing state. Free (no gas required).
- Check token balances
- View protocol parameters
- Query prices or rates

Write Contract: Execute state-changing functions. Requires wallet connection and gas payment.
- Claim rewards
- Emergency withdrawals
- Direct protocol interaction

Pro Tip: Use "Read as Proxy" and "Write as Proxy" for upgradeable contracts (those using proxy patterns).

Token Balances: View all ERC20, ERC721, and ERC1155 tokens held by any address

Token Transfers: Complete history of token movements in/out of the address

Token Analytics:
- Holder count and distribution
- Total supply and circulating supply
- Recent transfers and volume

NFT View: For NFT contracts, explorers show collection metadata, individual token images, and ownership history

Etherscan Family: The gold standard for EVM chain exploration. Etherscan operates explorers for multiple chains:
- Etherscan: Ethereum mainnet
- Arbiscan: Arbitrum One
- Optimistic Etherscan: Optimism
- Basescan: Base
- PolygonScan: Polygon PoS
- BscScan: BNB Chain

Consistent Interface: Same features and UI across all chains - learn one, use all

Premium Features: API access, advanced analytics, address labels, token verification

Why Open Source Matters: Open-source explorers can be self-hosted, modified, and are not dependent on a single company. Critical for decentralization.

Blockscout: Most widely used open-source explorer. Used by Optimism, Gnosis Chain, and hundreds of other networks. Features:
- Full source code on GitHub
- Self-hostable
- Active development community
- Advanced smart contract interaction

Routescan: Unified interface for multiple chains with focus on cross-chain analytics

Unified Search: Multi-chain explorers let you search across multiple blockchains simultaneously

Blockchair: Supports 40+ blockchains including Bitcoin, Ethereum, and many altchains. Features privacy mode to prevent address tracking.

OKLink: OKX's multi-chain explorer with institutional-grade analytics and API

EVMExplorer: Focused on EVM chains with unified interface across all compatible networks

Use Case: Tracking cross-chain activity, comparing metrics across chains, or when you don't know which chain a transaction is on

Different Architectures: Non-EVM chains have fundamentally different data structures requiring specialized explorers

Solana (Solscan): Programs instead of contracts, parallel transaction processing, unique account model

Cosmos (Mintscan): IBC transfers, validator sets, governance proposals, staking info

NEAR (Nearblocks): Sharded architecture, human-readable account names, access keys

TON (TONScan): Workchains, jettons (tokens), TON DNS names

Tron (TronScan): Energy/bandwidth system, TRC20 tokens, Super Representatives

Sui (SuiScan): Object-centric model, Move language, parallel execution

Cardano (CardanoScan): UTxO model, native tokens, stake pools

What Is Transaction Tracing?: Tracing replays a transaction step-by-step, showing every opcode execution, internal call, and state change.

samczsun's Transaction Tracer: Built by renowned security researcher @samczsun. Shows complete call hierarchy, storage reads/writes, and event emissions. Essential for understanding complex DeFi transactions.

Tenderly: Full debugging platform with transaction simulation, tracing, and alerting. Allows forking mainnet state for testing.

Use Cases:
- Debugging failed transactions
- Understanding exploit mechanics
- Analyzing MEV bundle execution
- Protocol research

The ABI Problem: Transaction input data is encoded bytes. Without the ABI (Application Binary Interface), you can't understand what function was called.

AnyABI: Aggregates ABIs from multiple sources (Etherscan, Sourcify, 4byte.directory). Even decodes transactions for unverified contracts using function signature databases.

4byte.directory: Database of function signatures. The first 4 bytes of calldata identify the function - this database maps them to human-readable names.

Pro Tip: If a contract isn't verified but you have a similar verified contract, you can often decode the transaction using that ABI.

Making Blockchain Social: Some explorers focus on translating complex transactions into plain English.

Once Upon: Transforms transactions into social media-style posts. Instead of showing raw data, it describes what happened: "vitalik.eth bought 10 ETH worth of USDC on Uniswap".

Benefits:
- Accessible to non-technical users
- Quick understanding of activity
- Social sharing of transactions

Limitations: May oversimplify complex transactions. Use traditional explorers for detailed analysis.

Exchange Deposits:
1. Copy your deposit address from the exchange
2. Paste into explorer and watch for incoming transactions
3. Check confirmations required by exchange
4. Transaction appears in exchange once confirmed

Withdrawal Verification:
1. Exchange provides withdrawal transaction hash
2. Search hash on explorer to verify it was broadcast
3. Monitor confirmations until finalized
4. Verify destination address received funds

Pro Tip: Bookmark your exchange's hot wallet addresses to monitor overall exchange activity

Step 1: Find the Error
On the transaction page, look for the revert reason. Common errors:
- "UniswapV2: INSUFFICIENT_OUTPUT_AMOUNT" = Slippage exceeded
- "ERC20: transfer amount exceeds balance" = Insufficient tokens
- "execution reverted" = Generic failure, check trace

Step 2: Analyze the Trace
Use the "State" tab or a tracer to see exactly where execution failed.

Step 3: Common Fixes
- Increase slippage tolerance
- Approve tokens before swap
- Check token balances
- Verify contract address is correct
- Ensure sufficient gas limit

Before Interacting with Any Project:

1. Check Contract Verification
- Is the source code verified? Unverified = red flag
- Does the code match what they claim?

2. Review Contract Activity
- How old is the contract?
- Transaction volume and patterns
- Any suspicious large withdrawals?

3. Check the Team
- Look up deployer address
- What other contracts have they deployed?
- Any connection to known entities?

4. Token Analysis
- Is liquidity locked?
- Token distribution (concentrated holdings = risk)
- Honeypot detection

Finding Whale Wallets:
- Check top holders on token pages
- Monitor large transfers in real-time
- Track labeled addresses (exchanges, funds, known individuals)

Analyzing Whale Behavior:
- What tokens are they accumulating?
- DeFi protocols they're using
- Transaction patterns and timing

Research Applications:
- Alpha discovery: What are smart money traders buying?
- Risk assessment: Are insiders dumping?
- Market intelligence: Exchange inflow/outflow trends

Tools: Combine explorer data with analytics platforms like Nansen, Arkham, or DeBank for labeled address insights

Capabilities:
- Account balances and transaction history
- Contract ABIs and source code
- Token information and transfers
- Gas price oracle
- Block and transaction data

Rate Limits:
- Free tier: 5 calls/second
- Pro: Up to 20 calls/second
- Enterprise: Custom limits

API Key: Required for all endpoints. Register free at etherscan.io

Example Use Cases:
- Portfolio trackers
- Tax reporting tools
- Automated alerts
- DeFi dashboards

Rate Limit Strategies:

1. Caching
- Cache responses that don't change often
- Store historical data locally
- Use TTL appropriate to data type

2. Batching
- Combine multiple requests where possible
- Use multi-address endpoints
- Aggregate queries efficiently

3. Fallbacks
- Use multiple explorers' APIs
- Switch between Etherscan, Blockscout, etc.
- Implement exponential backoff

4. Consider Alternatives
- Run your own node for heavy usage
- Use The Graph for custom queries
- Consider paid RPC providers

Blockscout API:
- Open-source, self-hostable
- GraphQL and REST endpoints
- No registration required for public instances

The Graph:
- Decentralized indexing protocol
- Custom subgraphs for any protocol
- GraphQL queries
- Used by major DeFi protocols

Direct RPC:
- Query nodes directly
- Full control over data access
- Requires more technical setup
- Providers: Infura, Alchemy, QuickNode

Choosing the Right Option:
- Simple queries: Explorer APIs
- Complex/custom: The Graph
- Real-time/historical: Direct RPC
- High volume: Self-hosted infrastructure