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