In the last five years there has been cumulative evidence showing that massivegalaxies have dramatically grown in size since z ˜ 3. This result hasremained very controversial as it seems at odd with our previous knowledgebased on the detailed analysis of the stellar populations of nearby massivegalaxies which shows that their stars were form very early on and over a shorttime interval. In this contribution, I will summarize what we have learned since the discovery of these compact objects, the mechanisms proposed to explaintheir size increase and the future research lines to explore.