The Midi-Chlorians are a sapient species of microscopic symbionts which inhabit the cells of all living (organic) creatures and allow them to be connected to the Force. According to Jedi Knight Qui-Gon Jinn, life couldn't exist without the Midi-Chlorians. The Jedi further claims that they "continually speak" to their hosts, communicating the Force's will, and allowing those who are capable of listening to them to unlock the innate powers of the Force.
It is believed that the concentration of Midi-Chlorians within one's cells is related to one's sensitivity to the Force. For this reason, the Jedi would analyse blood samples from Force-sensitive individuals to determine their Mini-Chlorian count. The highest count ever observed was in Anakin Skywalker, a Human boy who was conceived without a father, leading the Jedi to speculate that he might have been conceived by the Midi-Chlorians themselves as the prophetic "Chosen One", destined to restore balance between the light-side and the dark-side of the Force.
- Although Midi-Chlorians were first mentioned in the 1999 film Star Wars: Episode I - The Phantom Menace, the concept behind them was created by George Lucas in 1977, although it ended up being left out from the original Star Wars trilogy.
- Midi-Chlorians are vaguely based on real life mitochondria: cellular organelles which originated from symbiotic bacteria and inhabit the cells of eukaryotes such as plants, animals, fungi and protozoans. Like the Midi-Chlorians, their existence is vital to their hosts, so, as Qui-Gon would put it, "life could not exist" without the mitochondria (at least not eukaryote life).
- In 2006, a species of minuscule bacteria living inside the mitochondria of the cells of castor bean ticks (Ixodes ricinus) was officially given the scientific name Midichloria mitochondrii in homage of the Midi-Chlorians and their creator George Lucas.