One of the key, eco-friendly aspects of writing with a fountain pen is the ability to refill it. Instead of dipping the nib in a nearby bottle of ink, the modern fountain pen holds ink inside the barrel in a variety of methods. Watch the video link below to see how to operate the most common fountain pen filling mechanisms.

The most common fountain pen filling systems are:

Ink cartridge
Cartridge converter
Internal piston filler
Vacuum filler
Eyedropper filler

The type of filling system that best suits you is a decision that weighs convenience versus ink capacity. If you want a high ink capacity and don't switch colors often, then a piston, vac or eyedropper will work well for your writing habits. A writer that lives out of their suitcase might find cartridges more convenient than carrying glass bottles of ink. If you change colors often and have 10 to 20 pens inked at one time, you may like the option of a cartridge converter so your pens do not sit with a full barrel of ink for months at a time. 

Other exotic / vintage filling mechanisms include:

Lever, crescent, vacumatic, safety, snorkel, aerometric, touchdown, sleeve filler, blow filler, and pneumatic filler, etc.