An ink cartridge or inkjet cartridge is a replaceable component of an inkjet printer that contains the ink (and sometimes the print head itself) that is deposited onto paper during printing.

Each ink cartridge contains one or more partitioned ink reservoirs; certain manufacturers also add electronic contacts and a chip that communicates with the printer.

Because printer cartridges from the original manufacturer are often expensive, demand exists for cheaper third party options. These include ink sold in bulk, cartridge refill kits, machines in stores that automatically refill cartridges, remanufactured cartridges, and cartridges made by an entity other than the original manufacturer.

Consumers can refill ink cartridges themselves with a kit, or they can take the cartridge to a refiller or remanufacturer where ink is pumped back into the cartridge. PC World reports that refilled cartridges have higher failure rates, print fewer pages than new cartridges, and demonstrate more on-page problems like streaking, curling, and color bleed.

Another option is for the consumer to purchase "bulk ink" (in pints, quarts, or gallons) and refill the cartridges themselves. This can be extremely cost-effective if the consumer is a heavy user of cartridges. 1 US pint (0.47 l; 0.83 imp pt) is sufficient to fill about 15 to 17 large-capacity cartridges.

Generally speaking, Canon, Dell, HP, and Lexmark cartridges are not difficult to refill, though some Lexmark cartridges employ a built-in counter chip that can't be reset; Epson cartridges also have a built-in counter chip, however it is possible to purchase a chip resetter. As Brother cartridges generally lack any chip and consist of merely a sack of ink, they can be readily refilled.[citation needed] Since refilling involves handling ink, it can be a messy process for inexperienced people.

Some third party manufacturers have been offering refillable cartridges with an auto reset chip to simplify the refilling process. These refillable cartridges are less harmful to the environment and often easy to further refill.

Laser toner or inkjet cartridges sold as "remanufactured" are usually re-filled cartridges, although many third party newly manufactured "compatible" cartridges exist. Inkjet cartridges sold as "compatible" are typically newly manufactured cartridges. Inkjet cartridges sold as "remanufactured" are cartridges that have been used at least once by a consumer and then refilled by a third party.

The legality of this industry was brought to the United States Court of Appeals for the Sixth Circuit in the case of Lexmark Int'l v. Static Control Components. The Court ruled that reverse-engineering the handshaking procedure to enable compatibility did not violate the Digital Millennium Copyright Act.

HP has fiercely defended its printing interests from the refill industry, including filing patent complaints and false advertising lawsuits which allege that inferior ink is not properly differentiated from the original HP ink.