A novel approach is presented to modeling action and change within the Situation Calculus in the presence of both non-determinism and probabilistic behavior. Two examples are given to illustrate the approach: the Russian roulette and the alternating bit protocol. P oss( send (m, b), Dlvrd , s) ⊃ holds(sent(m, b), do( send (m, b), Dlvrd , s)), P oss( ack (b), Dlvrd , s) ∧ holds(rbit(b), s) ⊃ ¬holds(sbit(b), do( ack (b), Dlvrd , s)). Action Precondition Axioms P oss [ (ack(b), s) ⊃ holds(rbit(b), s), P oss [ (send(m, b), s) ⊃ holds(sbit(b), s).