Colloquium Mathematical Philosophy, R. A. Rynasiewicz (Johns Hopkins University) gives a talk at the MCMP Colloquium (10 June, 2015) titled "On the Role of the Light Postulate in Relativity". Abstract: As presented by Einstein in 1905, the theory of special relativity follows from two postulates: first, what he called the principle of relativity, and second, an empirical fact about the relation of the propagation of light relative to its source that has come to be called the light postulate. In 1910 Waldemar von Ignatowsky claimed to be able to derive the Lorentz transformations, and hence special relativity, without the light postulate using only the principle of relativity and assumptions that Einstein seems to have implicitly made, such as linearity and the isotropy and homogeneity of space. In his authoritative Relativitätstheorie of 1921, Pauli dismissed Ignatowsky’s result without explanation as void of physical significance. More recently, respected physicists and foundationalists, such as David Mermin (1984), have defended Ignatowsky and claimed that special relativity pre- supposes nothing about electromagnetism. In the first part of this talk, I discuss just what the light postulate asserts (both in special and in general relativity). In the second, I hope to shed light on the debate, if not definitively settle it. (To say on which side would spoil the suspense.) I will also discuss related attempts to dismiss the conventionality of simultaneity.