The specification generally relates to compounds of Formula (I): and pharmaceutically acceptable salts thereof, where Q, R1, R2, R3, R4 and R5 have any of the meanings defined herein. The specification also relates to the use of such compounds and salts thereof to treat or prevent ATM kinase mediated disease, including cancer. The specification further relates to crystalline forms of compounds of imidazo[4,5-c]quinolin-2-one compounds and pharmaceutically acceptable salts thereof; pharmaceutical compositions comprising such compounds and salts; kits comprising such compounds and salts; methods of manufacture of such compounds and salts; intermediates useful in the manufacture of such compounds and salts; and to methods of treating ATM kinase mediated disease, including cancer, using such compounds and salts.