Unlike ballpoint pens, fountain pens contain a lot of detail and expensive craftsmanship in the nib. Thus, nearly all fountain pens are designed to be refillable. A number of different refill mechanisms exist, including piston fillers, sac fillers, lever fillers, and cartridge-converter fillers.

Fountain pen ink cartridges date back to the 1950s. Some early models were made out of glass, but various plastics quickly became the material of choice. Some manufacturers experimented with refillable cartridges, but these were messy and unpopular; disposable cartridges quickly became the norm.

Cartridges are considered by some to be more convenient than bottled ink. This depends a lot upon the pen's filling mechanism — whilst a good cartridge pen is less prone to mess than a bad bottle filler, a modern piston filler is very clean. Cartridges may also be easier to carry around than some ink bottles.
There are downsides to cartridges too. They are typically more expensive than bottled ink and are available in fewer colours and styles. The ink flow from cartridges tends to be slower, which can be a problem with some nibs. Also, pens designed to take cartridges carry less ink than a dedicated sac filler of similar dimensions compared to fountain pens.