What Is an Automated Teller Machine?

An automated teller machine, commonly abbreviated ATM, is an electromechanical self-service terminal that lets a cardholder perform banking transactions without a human teller. A typical unit authenticates the customer by combining something they hold, a payment card, with something they know, a personal identification number, then contacts the card issuer over a financial network to authorize the requested transaction before dispensing cash, accepting a deposit, or updating an account balance. The machine is at once a mechanical device that must count and move banknotes accurately, a secure cryptographic endpoint, and a networked transaction terminal.

The ATM has no single inventor. Cash dispensing machines appeared in London in 1967, Chemical Bank installed the first United States unit at Rockville Centre, New York in 1969, and engineers at Chubb, Docutel, NCR, Diebold, Burroughs, Omron Tateishi, and IBM each contributed pieces of the design during the following decade. Magnetic stripe encoding on plastic cards and online authorization against a central account file were the two developments that turned a standalone dispenser into a banking terminal.

Hardware Subsystems

An ATM is built around a currency dispenser mounted inside a certified safe. Cassettes hold banknotes of one denomination each, and a picker mechanism using friction rollers or vacuum feed separates single notes, passes them through a thickness and double-detect sensor stage, and stacks the counted bundle at the presenter. Rejected notes divert to a purge bin so a miscount never reaches the customer. Deposit-capable machines add a bunch note acceptor that scans each note for denomination, authenticity, and fitness, and either recycles it into a dispensing cassette or stores it in an escrow. The remaining subsystems are the card reader, which may be motorized or dip-type and increasingly reads an EMV chip or a contactless interface, an encrypting PIN pad, a receipt printer, a journal record, and the operator display. Anti-skimming shutters, card jitter mechanisms, and vibration and gas sensors on the safe address physical attacks.

Transaction Networks and Message Standards

When a transaction is initiated, the terminal composes an authorization request and sends it to an acquirer, which routes it through a shared switch to the card issuer. The message format is defined by ISO 8583, the interchange message specification for financial transaction card originated messages, which fixes the message type indicator, the bitmap that declares which data elements are present, and the encoding of each element such as primary account number, processing code, amount, and terminal identifier. Interbank switches use these messages to route across networks and to settle balances between institutions afterward. Terminal software historically ran on proprietary platforms and now typically runs a hardened commercial operating system beneath a vendor application layer, with remote monitoring for cassette levels, device faults, and software distribution.

Security and Cryptography

The PIN is never handled in the clear outside a tamper-responsive boundary. It is encrypted inside the PIN pad, carried as a PIN block, and translated between keys only within hardware security modules at each switching node. Key management historically relied on the Triple Data Encryption Algorithm specified in NIST Special Publication 800-67, which was withdrawn at the start of 2024 as the industry moved to the Advanced Encryption Standard. Chip authentication under the EMV specifications replaced static magnetic stripe data with a dynamic cryptogram computed per transaction, which removed the economics of card cloning at compliant terminals. Remaining attack classes include card trapping, cash trapping, physical jackpotting through the dispenser interface, and network-level attacks on the switch.

Applications

Automated teller machine technology has applications in areas including:

• Retail banking cash withdrawal, deposit, and account servicing
• Cash recycling in branches to reduce armored transport frequency
• Financial inclusion through white-label and agent banking deployments
• Currency exchange and cross-border card acceptance
• Bill payment, mobile top-up, and government disbursement terminals
• Cardless withdrawal using one-time codes or mobile credentials