Blockchain developer interviews test how distributed ledgers actually work — consensus, cryptography, smart contracts (usually Solidity on Ethereum), and the unique security concerns of immutable, money-handling code. Here are the blockchain developer interview questions that actually get asked.
Blockchain fundamentals
- How does a blockchain work — blocks, hashes, the chain?
- What makes it immutable and decentralized?
- Consensus mechanisms — Proof of Work vs Proof of Stake.
- Public vs private vs consortium blockchains.
- What is a hash function and a digital signature here?
Smart contracts & Solidity
- What is a smart contract?
- Solidity basics — data types, mappings, modifiers, events.
- What is gas and why does it exist?
- The EVM; what happens when a transaction runs.
- ERC-20 and ERC-721 (tokens and NFTs).
Security (critical)
- The reentrancy attack and the checks-effects-interactions pattern.
- Integer overflow/underflow; common vulnerabilities.
- Why is smart contract code so hard to fix after deployment?
- Gas optimization.
How to prepare
Blockchain rounds probe the model and security verbally. Practise explaining consensus and reentrancy out loud. Greenroom runs spoken technical interviews that follow up on your reasoning. Pair it with our backend guide.
Frequently asked questions
What questions are asked in a blockchain developer interview?
Blockchain developer interviews cover how blockchains work (blocks, hashes, immutability, decentralization), consensus mechanisms (Proof of Work vs Proof of Stake), public vs private blockchains, cryptography (hash functions, digital signatures), smart contracts and Solidity (data types, mappings, modifiers, events, the EVM), gas, token standards (ERC-20, ERC-721), and security vulnerabilities like reentrancy and integer overflow.
What is the difference between Proof of Work and Proof of Stake?
Proof of Work secures the network by having miners compete to solve computationally expensive puzzles, with the winner adding the next block — secure but energy-intensive. Proof of Stake selects validators to propose and attest blocks based on the amount of cryptocurrency they stake as collateral, which is far more energy-efficient and is what Ethereum now uses. Both prevent malicious actors from cheaply rewriting history.
What is a reentrancy attack in smart contracts?
A reentrancy attack happens when a contract makes an external call before updating its own state, allowing the called contract to call back into the original function repeatedly and, for example, drain funds before balances are updated. It's prevented with the checks-effects-interactions pattern (update state before external calls) and reentrancy guards. It's the most famous smart-contract vulnerability and a near-certain interview topic.
How should I prepare for a blockchain developer interview?
Understand blockchain fundamentals, consensus, smart contracts and Solidity, gas and the EVM, but weight security heavily — reentrancy, the checks-effects-interactions pattern and common vulnerabilities — since immutable code holding value makes bugs catastrophic. Practise explaining consensus and reentrancy out loud with a voice-based mock interview that follows up on your reasoning.