  These notes focus on the description of the phases of matter in two
dimensions. Firstly, we present a brief discussion of the phase diagrams of
bidimensional interacting passive systems, and their numerical and experimental
measurements. The presentation will be short and schematic. We will complement
these notes with a rather complete bibliography that should guide the students
in their study of the development of this very rich subject over the last
century. Secondly, we summarise very recent results on the phase diagrams of
active Brownian disks and active dumbbell systems in two dimensions. The idea
is to identify all the phases and to relate, when this is possible, the ones
found in the passive limit with the ones observed at large values of the
activity, at high and low densities, and for both types of constituents.
Proposals for the mechanisms leading to these phases will be discussed. The
physics of bidimensional active systems open many questions, some of which will
be listed by the end of the Chapter.
