Let G be a connected reductive group over an algebraically closed field k, and let C be a smooth projective curve over k. I'll describe the Picard group of the moduli stack of principal G-bundles on C and explain consequences for the existence of Poincare families on the corresponding coarse moduli scheme, and for the Picard group of the latter. This is joint work with I. Biswas (TIFR, Mumbai).