Treballs Finals de Grau de Matemàtiques, Facultat de Matemàtiques, Universitat de Barcelona, Any: 2016, Director: Carles Rovira EscofetIn this undergraduate thesis we will introduce the branching processes and one of their major applications, the neutron branching processes (one-group, isotropic case). First, in chapter I we begin with Galton-Watson processes and their properties. Chapter II deals with a generalization of branching processes introducing a parameter x (type) for each object of the processes that may represents, energy, position age etc. Finally in chapter III we apply this generalization into a real case, the neutron branching processes