It is well known that when a fluid is thermally unstable, convection can be the most efficient mechanism for mixing and energy transport. However, the presence of a stable composition gradient can counteract and inhibit convection, giving rise to an interesting phenomenon called double-diffusive convection - or semi-convection - which is characterized by layers of convective and diffusive zones. In this seminar, I will discuss some of the main properties of semi-convection and its relevance to gas giant planets. Finally, I will show 2D hydrodynamical simulations of this process.