The Kohler theory of activation is compared with classical nucleation theory for the NaCl-H2O system. We find that for NaCl particles of the size typically found in the atmosphere the two processes are hardly distinguishable. Consequently, the usually preferred choice of simple activation theory over nucleation in the development and application of large scale atmospheric models, is reaffirmed. However, we also find that for small NaCl crystals consisting of 10-50 molecules, nucleation can occur at relative humidities significantly lower than those required for activation.