Proto-planetary discs provide the gaseous and solid building blocks to assemble planets. Therefore, only if we understand proto-planetary discs can we build a successful planet formation theory. In the last few years new instruments, especially the sub-mm interferometer ALMA, have allowed us to conduct entire surveys of proto-planetary disc populations, giving us a chance to probe the inventory of planet-forming material. I will recap the main findings of these surveys. I will discuss in particular how the field is critically reconsidering the viscous framework normally used to describe proto-planetary discs. I will discuss how to theoretically interpret the findings of the surveys in the viscous framework and in the competing MHD wind framework, and look at the outlook in the future of how observations may be able to distinguish between the two alternatives.