In this talk I will introduce a particular kind of irrelevant deformation of 2D QFTs, known as TT deformation and review some of the results obtained recently on this subject. The general tone will be pedagogical and intended for an audience of nonexperts. After defining the operator TT and describing its main properties, I will present some motivations which justify the interest in these peculiar deformations. I will then move on to the derivation of important non-perturbative results and describe the principal features of the deformed theories. Finally I will display some of the most recent results and a number of interesting questions that still wait for answers.