Transition systems are focal structures in the study of concurrent systems. On the one hand they are used for defining operational semantics of such systems. And on the other hand they are fundamental structures for interpreting modal and temporal logics. Here we consider different transition systems associated with Milner's Calculus of Communicating Systems (CCS), these differ according to how silent actions are treated. Then a general framework for modal and temporal logics is outlined. Within this framework modal and temporal mu-calculi are highlighted, logics that are appropriate for describing CCS processes. Finally, the equivalences induced by these logics on processes is examined, in general terms.