There can be several instances in the early universe where a scalar field condensate decays in a violent and highly inhomogeneous way. A major example is the process of preheating after inflation, when the particle content of the universe is produced by the decay of the inflaton and then evolve towards the thermal bath of the Hot Big Bang. These phenomena generate stochastic backgrounds of gravitational waves, which carry unique informations about their dynamics and the underlying high-energy physics, and which could be observable by interferometric experiments like LIGO/VIRGO and LISA. In this talk, I will briefly review the methods that we have developed to compute the gravity wave spectrum produced by such cosmological sources and the results that have been obtained so far. In particular, I will discuss the gravitational wave signals emitted by preheating after hilltop inflation and by the non-perturbative decay of flat direction condensates in supersymmetric theories. I will then consider models where not only scalar fields, but also gauge fields play an important role. I will show that they leave specific signatures in the gravity wave spectrum, notably through the production of cosmic strings configurations.