The recent interest in multiferroics, materials exhibiting simultaneous magnetic and ferroelectric order, is motivated in part by the cross-coupling between magnetic and electronic degrees of freedom in these materials. This magnetoelectric coupling allows the magnetic structure to be changed using an electric field or, conversely, the electric polarization to be switched by a magnetic field. I will discuss a number of different materials in which low temperature magnetic transitions produce a spontaneous electric polarization, focusing on systems having a single type of magnetic transition metal ion. This behaviour can be qualitatively understood using a Ginzburg Landau mean field approach, which incorporates higher order magnetoelectric terms coupling the magnetic and ferroelectric order parameters. This coupling between the magnetic and ferroelectric properties can be investigated experimentally by measurements on thin film samples. I will conclude with some brief comments about what we can learn about the microscopic coupling between charge, spin, and lattice degrees of freedom in these fascinating materials.