We live in a fairly stable and peaceful epoch for the Universe, and indeed our Galaxy appears to be evolving gently and steadily, forming stars at the relatively modest rate of  ~3 M⊙yr-1. However, we know that 10 billion years ago the Universe was a much more active place, with stars being formed at a 10 times higher rate than it is seen around us today. What physical processes are driving this dramatic change and their relative importance represents one of the unanswered questions of modern astrophysics. To understand this evolution one valuable way is to consider galaxies as ecosystems, in which the gas flows in and out of the galaxy, it is transformed into stars and eventually ejected back or recycled to form a new generation of stars. These processes are known as the “galaxy baryon cycle”. Understanding what rules the cold gas accretion rate into galaxies, the efficiency of converting baryons into stars and the role of outflowing gas in preventing the infall of new accreting material, has assumed increasing importance in galaxy formation studies.