The most remarkable recent discovery in fundamental physics is that the Universe is undergoing accelerated expansion. To achieve a proper understanding of its physical origin forces us to make a hard choice between dynamical and enviromental scenarios. The first approach predicts the existence of a new long distance physics in the gravitational sector, while the second relies on the existence of the vast landscape of vacua with different values of the cosmological constant. I will discuss achievements and shortcomings of each of the approaches, and illustrate them in the concrete examples.