In the last five years there has been cumulative evidence showing that massive galaxies have dramatically grown in size since z~3. This result has remained very controversial as it seems at odd with our previous knowledge based on the detailed analysis of the stellar populations of nearby massive galaxies which shows that their stars were formed very early on and over a short time interval. In this talk, I will summarize what we have learned since the discovery of these compact objects, the mechanisms proposed to explain their size increase and the future research lines to explore.