Lipschitz functions play the role of smooth functions in analysis on metric measure spaces, being used to approximate functions from various Sobolev spaces, of Hajlasz and Newtonian type. This approximation allows the extension of some properties, e. g. the validity of a Poincar´e inequality, from Lipschitz functions to Sobolev-type functions. We summarize several results on the density of Lipschitz functions in Sobolev-type spaces on metric measure spaces, progressing from the classical case of Sobolev spaces of first order, on Euclidean domains, to the case of Sobolev-type spaces based on rearrangement invariant Banach function spaces, on metric measure spaces.