p-adic Hodge Theory deals with p-adic representations of p-adic Galois groups. Iwill present the results of my thesis on how to compute with objects involved in this theory. I will briefly recall notions on Galois representations with coefficients in finite fields. I will then present our main tool to study them: the theory of phi-modules. I will describe an algorithm for the reduction of phi-modules and explain how it can be used to compute invariants of Galois representations.