We review the use of nondeterminism and identify two different purposes. The descriptive purpose handles uncertainties in the behaviour of existing entities. The constraining purpose is used in speci cations to constrain implementations. For the speci cation of embedded systems we suggest a stochastic process instead of nondeterminism. It serves mostly the descriptive purpose but can also be used to constrain the system. We carefully distinguish di erent interpretations of these concepts by the di erent design activities simulation, synthesis and veri cation.