An approach to the semantics of CCS-like communicating processes is 
proposed that is based upon evaluation of processes to input- or 
output-committed form, with no explicit mention of silent actions.  This 
leads to a co-inductively defined notion of evaluation bisimilarity---a 
form of weak branching-time equivalence which is shown to be a congruence, 
even in the presence of summation.  The relationship between this 
evaluation-based approach and the more traditional, labelled transition 
semantics is investigated.  In particular, with some restriction on sums, 
CCS observation equivalence is characterised purely in terms of evaluation 
to committed form, and evaluation bisimilarity is characterised as a weak 
delay equivalence.  These results are extended to the higher order case, 
where evaluation bisimilarity coincides with Sangiorgi's weak context 
bisimilarity.  An evaluation-based approach to pi-calculus and the 
relationship with Milner and Sangiorgi's reduction-based notion of barbed 
bisimulation are also examined.