A cash register is a mechanical or electronic device for calculating and recording sales transactions, and an attached cash drawer for storing currency. The cash register also usually prints a receipt for the customer.

In most cases the drawer can be opened only after a sale, except when using a special key, which only senior personnel or the owner has. This reduces the risk of personnel stealing from the shop owner by not recording a sale and pocketing the money, in the case that the customer does not require a receipt and has to be given change (cash is more easily checked against recorded sales than inventory). In fact, cash registers were first invented for the purpose of eliminating employee theft or embezzlement. The first registers were entirely mechanical, without receipts. The employee was required to ring up every transaction on the register, and when the total key was pushed, the drawer opened and a bell would ring, alerting the manager to a sale taking place. Those original machines were nothing but simple adding machines. Some cash registers include a key labeled "NS" (most of the time silver in color), which is abbreviated for "No Sale", and opens the drawer, printing out a receipt stating "No Sale" and recording it in the register log that the register was opened. Some other cash registers require a numeric password to be entered when attempting to open the register. In the event of a power shortage, some models have latches that can be pulled to open the drawer.

A cash register receipt may be compulsory for tax purposes. The law sometimes also requires customers to collect the receipt and keep it at least for a short while after leaving the shop,^{[citation needed]} again for checking that the shop records sales, so that it cannot evade taxes.

Often cash registers are attached to scales, barcode scanners, checkstands, and debit card or credit card terminals. Increasingly, dedicated cash registers are being replaced with general purpose computers with POS software.

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