  Observational studies of AGN in the mid-infrared regime are crucial to our
understanding of AGN and their role in the evolution of galaxies. Mid-IR-based
selection of AGN is complementary to more traditional techniques allowing for a
more complete census of AGN activity across cosmic time. Mid-IR observations
including time variability and spatially resolved imaging have given us unique
insights into the nature of the obscuring structures around AGN. The wealth of
fine structure, molecular, and dust features in the mid-IR allow us to
simultaneously probe multiple components of the ISM allowing us to explore in
detail the impact on the host galaxy by the presence of an AGN -- a crucial
step toward understanding galaxy-SMBH co-evolution. This review gives a broad
overview of this wide range of studies. It also aims to show the evolution of
this field starting with its nascency in the 1960s, through major advances
thanks to several generations of space-based and ground-based facilities, as
well as the promise of upcoming facilities such as the {\sl James Webb Space
Telescope (JWST)}.
