  Active systems are driven out of equilibrium by exchanging energy and
momentum with their environment. This endows them with anomalous mechanical
properties that we review in this colloquium for the case of dry scalar active
matter, which has attracted considerable attention. These unusual properties
lead to a rich physics when active fluids are in contact with boundaries,
inclusions, tracers, or disordered potentials. Indeed, studies of the
mechanical pressure of active fluids and of the dynamics of passive tracers
have shown that active systems impact their environment in non-trivial ways,
for example, by propelling and rotating anisotropic inclusions. Conversely, the
long-ranged density and current modulations induced by localized obstacles show
how the environment can have a far-reaching impact on active fluids. This is
best exemplified by the propensity of bulk and boundary disorder to destroy
bulk phase separation in active matter, showing active systems to be much more
sensitive to their surroundings than passive ones. This colloquium aims at
providing a unifying perspective on the rich interplay between active systems
and their environments.
