Expander graphs in general, and Ramanujan graphs in particular, have been a focus of a lot of research in the last five decades. Some of it originated in brain study! It has been probably the area with the most fruitful interaction between mathematics and computer science, where applications went both ways. In recent years a high dimensional theory of expanders has started to emerge. We will survey some of its challenges and achievements and try to explain why this is not just a generalization but also, an area that leads to new horizons in pure math (e.g. topological overlapping) and in CS ("property testing").