2D electron systems host interesting and exotic effects, aswell as provide possibilities of quantum devices of a new functionality. Based on the conventional 2D systems of GaAs 2DEG and graphene, many principles and phenomena have been found. Examples include the quantum Hall effects and topological matter, quantum electron transport, exotic quasiparticles, such as anyons and Majorana fermions, as well as quantum computations and simulations. Based on these findings, we are now at the stage of asking what the next key questionsand directionsare.