An ink cartridge or inkjet cartridge is a component of an inkjet printer that contains the ink that is deposited onto paper during printing. Each ink cartridge contains one or more ink reservoirs; certain producers also add electronic contacts and a chip that communicates with the printer. Most consumer inkjet printers, such as those made by Canon, HP, and Lexmark (but not Epson) use a thermal inkjet; inside each partition of the ink reservoir is a heating element with a tiny metal plate or resistor. In response to a signal given by the printer, a tiny current flows through the metal or resistor making it warm, and the ink in contact with the heated resistor is vaporized into a tiny steam bubble inside the nozzle.^[1] As a consequence, an ink droplet is forced out of the cartridge nozzle onto the paper. This process takes a fraction of a millisecond. The printing depends on the smooth flow of ink, which can be hindered if the ink begins to dry at the print head, as can happen when an ink level becomes low. Dried ink can be cleaned from a cartridge print head using isopropyl alcohol or water.^[2] Isopropyl alcohol will damage the printing head, IPA can dissolve the plastic at the connections and rubber gaskets. Tap water contains contaminants that may clog the print head, so distilled water and a lint-free cloth is recommended.^[3] The ink also acts as a coolant to protect the metal-plate heating elements − when the ink supply is depleted, and printing is attempted, the heating elements in thermal cartridges often burn out, permanently damaging the print head. When the ink first begins to run low, the cartridge should be refilled or replaced, to avoid overheating damage to the print head.^[4]

In earlier photocopiers, this low-cost carbon toner was poured by the user from a bottle into a reservoir in the machine^{[citation needed]}. Later copiers, and laser printers from the first 1984 Hewlett-Packard LaserJet,^[3] feed directly from a sealed toner cartridge. Laser toner cartridges for use in color copiers and printers come in sets of cyan, magenta, yellow and black (CMYK), allowing a very large color gamut to be generated by mixing.