A continuous ink system (CIS), also known as continuous flow system (CFS), or bulk feed ink system, is a cost-effective replacement for ink-jet printer cartridges. In contrast to conventional cartridges, a CIS/CFS uses very large (about 100ml/colour) ink tanks, which are connected with tubes to the print-head nozzles. The tanks can be refilled from an ink bottle or, sometimes a syringe. Many CIS systems have been developed to cover most Epson, HP, Canon, and other printer brands. These bulk feed systems also allow users to use inexpensive aftermarket inkjet inks as well as specialty inks for T-Shirt transfers, or Black and White inks for B&W printing. These options have allowed users to produce specialty prints at a fraction of the cost of purchasing special cartridges or using more expensive printing solutions.

A continuous ink supply system feeds ink cartridges by way of silicone tubing.

Continuous ink tank designs vary, most incorporate a constant level, barometric pressure design. Constant level CIS delivers a smooth flow of ink to the cartridges as needed. Front tank is vented to atmosphere, negative air pressure in the back tank delivers ink to the cartridges.