In the last decades it was shown that the stability of colloidal dispersions can be controlled by adding non-adsorbing polymers or other particles. Often the depletion-interaction is used as the tool to induce a controlled, tuneable attraction between colloidal particles in model systems]. In this talk an introduction to the basics of the depletion interaction and the resulting phase behaviour of colloidal mixtures is given, with a few illustrative novel results. Subsequently, examples are given on the relevance for crowding in cells, food science and technology, protein crystallization and concentration of bacteria.