  Reactive Turing machines extend classical Turing machines with a facility to
model observable interactive behaviour. We call a behaviour executable if, and
only if, it is behaviourally equivalent to the behaviour of a reactive Turing
machine. In this paper, we study the relationship between executable behaviour
and behaviour that can be specified in the pi-calculus. We establish that all
executable behaviour can be specified in the pi-calculus up to
divergence-preserving branching bisimilarity. The converse, however, is not
true due to (intended) limitations of the model of reactive Turing machines.
That is, the pi-calculus allows the specification of behaviour that is not
executable up to divergence-preserving branching bisimilarity. Motivated by an
intuitive understanding of executability, we then consider a restriction on the
operational semantics of the pi-calculus that does associate with every pi-term
executable behaviour, at least up to the version of branching bisimilarity that
does not require the preservation of divergence.
