In this talk I will present a general perspective on the topological properties and theta-angle dependence of gauge theories, mainly focusing on the finite temperature case. The main aim is to highlight the physical ideas involved and discuss some recent results obtained by using Lattice QCD simulations, giving some hints of the numerical problems but avoiding the lattice technicalities. Besides the theoretical interest of studying a completely non-perturbative aspect of QCD, this subject has also some direct phenomenological implications, being strictly connected to the physics of axions and to their possible role as dark matter candidates.