A mad scientist is generally a stock character, or a parody of a stock character, of popular fiction, more specifically Science Fiction. The mad scientist may be villainous, benign or neutral, and whether insane, eccentric, or simply bumbling, mad scientists often work with fictional technology in order to forward their schemes, if they even have a coherent scheme. Alternatively, they fail to see the evil that will ensue from the hubris of “playing god”. Not all mad scientists are evil or villains. Some are protagonists (or at least positive forces), such as Dexter in the animated series Dexter's Laboratory. Occasionally there are self parodies of mad scientists making fun of the stereotype.

Though the archetypes often overlap, a mad scientist need not be an evil genius. A mad scientist is simply a scientist who has become obsessively involved with their studies and has begun to develop eccentricities by normal standards; an evil genius is a genius who uses their gift for explicitly, consciously evil purposes. For example, while a mad scientist would test the bounds of science to create an army of zombies, he may do it to see if – or prove that – he could, or out of boredom, to impress women, to help clean up his house, or many other such reasons. By contrast, an evil genius would construct his army with a purpose, such as taking over the world – in addition to being evil, such characters tend to have large-scale ambition (see Megalomania in fiction). A mad scientist may be a naive pawn of an evil genius, the evil genius often promising the scientist the funds and resources to conduct his research. Mad scientists also, whilst definitely being intelligent, usually fail to think things through to their conclusion while an evil genius is usually a clever planner and would have a diabolical use for the army of zombies as well as a plan to avoid being killed by them.

Occasionally a Mad Scientist may be an Evil Genius, using their creations for evil reasons.