The trolley problem, first described by Foot (1967) and Thomson (The Monist, 59, 204–217, 1976), is one of the most famous and influential thought experiments in deontological ethics. The general story is that a runaway trolley is threatening the lives of five people. Doing nothing will result in the death of those persons, but acting in order to save those persons would unavoidably result in the death of another, sixth person. It appears that, depending on the situation, we have different (...) moral judgments about the permissibility of action. We will review and systematize all the proposals in the literature of the past 35 years that have attempted to grasp our moral intuitions in a simple deontological principle. In particular, seventeen proposals will be classified: six algorithmic, seven psychological, and four other invalid accounts. This review and classification sheds light on some subtle differences and clarify a few issues. (shrink)