In this talk, I will discuss numerical simulations of relativistic jets in massive stars in the context of the collapsar model for long soft gamma-ray bursts. The propagation and breakout of such jets are examined in multi-dimensional special relativistic hydrodynamical simulations. During its propagation, the jet is collimated by the passage through the stellar mantle. As it erupts, the highlyrelativistic jet core is surrounded by a cocoon of less energetic, but still moderately relativistic ejecta that expands and becomes visible at larger polar angles. Implications for both the observations and the models which generate the jets are discussed.