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.