The isocurvature component of the primordial perturbations has been largely ignored ever since CMB experiments established that on large-scales the adiabatic mode dominates. On small scales, however, isocurvature can still play a role. I present a new signal of small-scale isocurvature, the formation of primordial black holes, which I use to derive new model-independent isocurvature constraints. I then discuss the isocurvature we expect in the Standard Model from the evolution of the Higgs field during reheating.