  This thesis deals with the specification and construction of syntax and
operational semantics of a programming language. We work with a general notion
of signature for specifying objects of a given category as initial objects in a
suitable category of models.This characterization, in the spirit of Initial
Semantics, gives a justification of the recursion principle. Languages with
variable binding, such as the pure lambda calculus, are monads on the category
of sets specified through the classical algebraic signatures. The first
extensions to syntaxes with equations that we consider are quotients of these
algebraic signatures. They allow, for example, to specify a binary commutative
operation. But some equations, such as associativity, seem to remain out of
reach. We thus introduce the notion of 2-signature, consisting in two parts: a
specification of operations through a usual signature as before, and a set of
equations among them. We identify the class of algebraic 2-signatures for which
the existence of the associated syntax is guaranteed. Finally, we takle the
specification of the operational semantics of a programming language such as
lambda calculus with beta-reduction. To this end, we introduce the notion of
reduction monad and their signatures, then we generalize them to get the notion
of operational monad.

Logic in Computer Science