  Despite the diversity of materials designated as active matter, virtually all
active systems undergo a form of dynamic arrest when crowding and activity
compete, reminiscent of the dynamic arrest observed in colloidal and molecular
fluids undergoing a glass transition. We present a short perspective on recent
and ongoing efforts to understand how activity competes with other physical
interactions in dense systems. We first review recent experimental work on
active materials that uncovered both classic signatures of glassy dynamics and
intriguing novel phenomena at large density. We introduce a minimal model of
self-propelled particles where the competition between interparticle
interactions, crowding, and self-propulsion can be studied in great detail. We
discuss more complex models that include some additional, material-specific
ingredients. We end with some general perspectives on dense active materials,
suggesting directions for future research, in particular for theoretical work.
