 Most philosophers believe that, other things being equal, simpler theories are better. But what exactly does theoretical simplicity amount to? Syntactic simplicity, or elegance, measures the number and conciseness of the theory’s basic principles. Ontological simplicity, or parsimony, measures the number of kinds of entities postulated by the theory. One issue concerns how these two forms of simplicity relate to one another. There is also an issue concerning the justification of principles, such as Occam’s Razor, which favor simple theories. The history of philosophy has seen many approaches to defending Occam’s Razor, from the theological justifications of the Early Modern period, to contemporary justifications employing results from probability theory and statistics.