How Blockchain Technology Works

A blockchain is a shared digital ledger that is copied across many independent computers and updated only when they agree on the next set of records. Records are grouped into blocks, and each block is linked to the one before it with a cryptographic fingerprint, so changing past data would mean rewriting everything that came after. The short answer to the question "what is blockchain?" is therefore a database that no single party controls and that is very hard to alter once data has been accepted.

Below you will find blockchain technology explained step by step: how a transaction becomes part of a block, how the network agrees on the order of events, why old records are so difficult to change, and where the technology is used in crypto, finance and beyond.

Key takeaways

• A blockchain stores data in blocks linked by cryptographic hashes, and every full node keeps its own copy.
• New blocks are added through a consensus mechanism, most commonly proof-of-work (Bitcoin) or proof-of-stake (Ethereum since September 2022).
• Changing an old record on a large public blockchain would require controlling most of the network's computing power or staked funds.
• Confirmed blockchain transfers cannot be cancelled or charged back.
• Blockchain and cryptocurrency are related but not the same thing: blockchain is the infrastructure, and a cryptocurrency is one type of asset recorded on it.

The idea was first put into practice by Bitcoin. Its white paper, published under the name Satoshi Nakamoto in October 2008, described "a peer-to-peer electronic cash system" that could work without a bank, and the first block of the Bitcoin blockchain was created in January 2009. The problem it solved was double spending: how to stop someone from sending the same digital coin twice when there is no central authority keeping the books.

The solution was to give every participant a copy of the ledger and a common set of rules for adding to it. What is a blockchain in crypto, then? It is the public record of who owns which coins and every transfer between addresses since the network started. Anyone can download it, check it and see whether a payment has been made, which is why crypto transfers can be verified without trusting the person who sent them.

Since then, the same design has been adapted for other uses, from smart-contract platforms such as Ethereum to private ledgers run by banks and companies. All of them share three basic ingredients: blocks of data, cryptographic links between those blocks, and rules that let independent computers agree on a single version of history.

Every blockchain follows the same broad cycle, whether it records coin transfers or other kinds of data. So how does blockchain work in practice?

Follow a single crypto transfer from start to finish:

• Signing. You create a transaction in your wallet and sign it with your private key. The signature proves you control the funds without revealing the key itself.
• Broadcasting. The wallet sends the transaction to nodes on the network, which pass it on to their peers. Pending transactions wait in a pool known as the mempool.
• Validation. Nodes check that the signature is valid, that the funds exist and have not already been spent, and that the network's rules are followed.
• Block building. A miner or validator selects pending transactions, usually prioritising those paying higher fees, and packages them into a new block.
• Consensus. The network agrees that the block is valid and adds it to the chain.
• Confirmations. Each additional block built on top of yours counts as a confirmation and makes the transaction harder to reverse.

Sending from a crypto wallet follows exactly this process. With a self-custody wallet you sign the transaction yourself; with a custodial wallet, the provider signs it on your behalf.

A block has two parts: a header and a list of transactions. The header contains the hash of the previous block, a timestamp and a single hash, called the Merkle root, that summarises every transaction in the block. A hash is a fixed-length fingerprint produced by a cryptographic function, such as SHA-256 in Bitcoin or Keccak-256 in Ethereum. Change even one character of the input and the fingerprint changes completely. Because each header includes the previous block's hash, the blocks form a chain. Hashes, blocks and consensus are the blockchain technology basics; everything else builds on them.

Consensus is how thousands of independent nodes agree on which block comes next. The two main approaches compare as follows:

Other mechanisms exist, such as delegated proof-of-stake and the voting-based systems used in many private networks, but all of them aim to answer the same question: which version of the ledger is the true one?

To alter an old transaction, an attacker would have to change the block that contains it. That changes the block's hash, which breaks the link to the next block, so every later block would have to be rebuilt as well. On a proof-of-work network, the attacker would also need to produce those blocks faster than the rest of the network combined, which in practice means controlling more than half of its computing power. This is known as a 51% attack.

On large networks the cost makes such an attack impractical. On Ethereum, reverting a finalised block would require an attacker to accept the loss of at least one-third of all staked ETH. Smaller networks are more exposed: Ethereum Classic, for example, suffered three 51% attacks in August 2020, during which blocks were reorganised and some coins were spent twice. That is why exchanges require different numbers of confirmations for different coins before crediting a deposit.

The data is protected not because changing it is physically impossible, but because rewriting history in a way the rest of the network would accept is extremely expensive.

A blockchain is only as strong as the network that runs it. That network consists of nodes, the computers that store the ledger, check new transactions and blocks against the rules, and share data with each other. Full nodes keep a complete copy of the chain and verify everything independently; light clients, which most mobile wallets use, rely on full nodes for data; and miners or validators produce new blocks. No single node holds a master copy: each keeps its own and follows the version that the rules and consensus point to.

A useful way to answer "what is a blockchain network?" is to look at who is allowed to take part:

The honest answer to "what is the blockchain used for?" in 2026 is: mostly crypto and finance, with a growing number of projects elsewhere. Blockchain is most useful when several parties who do not fully trust each other need to share the same record, and when it matters that no one can quietly change it afterwards. Smart contracts, programs stored on a blockchain that run automatically when set conditions are met, extend this from record keeping to automated agreements.

For most people, the practical answer to "what is blockchain in cryptocurrency?" is that it is the settlement layer: the place where ownership of Bitcoin, Ether, stablecoins and other tokens is recorded and transferred. Exchanges watch the blockchain to credit deposits and broadcast withdrawals. When you exchange crypto to fiat, the platform usually waits for a set number of confirmations on your deposit before releasing pounds or euros. Not every crypto trade touches the blockchain, though: trades between customers of the same exchange are normally recorded in the exchange's own database, and only deposits and withdrawals are settled on-chain.

Traditional finance is also testing the technology for securities. In the EU, the DLT Pilot Regime (Regulation (EU) 2022/858), in force since 23 March 2023, allows market infrastructures to trade and settle securities on distributed ledgers under adjusted rules. In the UK, the Bank of England and the FCA opened the Digital Securities Sandbox in 2024 for firms testing digital securities depositories and trading venues.

Outside crypto, most projects are still pilots or limited deployments, but several areas stand out:

• Supply chains: recording each hand-off of goods so their origin and route can be checked later, for example for food safety or sustainability claims.
• Public sector and credentials: the European Blockchain Services Infrastructure (EBSI), run by the European Commission and EU countries, supports verifiable credentials such as digital diplomas that can be checked across borders.
• Healthcare: tracking medicines through the supply chain and testing patient-controlled access to medical records.
• Trade finance: sharing bills of lading, letters of credit and other documents between banks, shippers and buyers to reduce paperwork and fraud.
• Land and asset registries: testing tamper-evident records of ownership.

So far, the answer to "what is the impact of blockchain on businesses?" is more modest than the hype of the late 2010s suggested. The technology works well for specific problems, mainly shared records between organisations and the settlement of digital assets, but it is rarely a replacement for an ordinary database.

Data protection is a particular issue in the UK and EU. The GDPR gives people the right to have their personal data erased, which is hard to reconcile with a ledger designed to be permanent. In 2025, the European Data Protection Board published draft guidelines on processing personal data through blockchain technologies, and the usual advice is to keep personal data off-chain and store only references or proofs on the blockchain.

The two terms are related but not interchangeable. Blockchain is the infrastructure; a cryptocurrency is one of the things that can be recorded on it. Public blockchains usually have a native coin, such as BTC or ETH, because it is used to pay transaction fees and reward the people who secure the network, while private blockchains often work without one.

This article is for general educational purposes and does not constitute financial, investment, legal or tax advice. Cryptoassets are high-risk, their value can fall to zero, and blockchain transfers are generally irreversible. In the UK, cryptoassets are generally not protected by the FSCS, and in the EU they are not covered by deposit guarantee schemes.