I review the experimental and theoretical situation regarding two problems connected with the transition between the superfluid A and B phases of liquid 3He, namely (I) what is the mechanism of nucleation of the strongly hypercooled first order transition between the two phases, (2) what factors govern the expansion of the stable B phase once formed, i.e. what determines the mobility of the A-B interface?It is concluded that in both cases there are a number of intriguing questions which at present remain unanswered.