The principal divisions of the town are Effretikon, Illnau, Ottikon and Bisikon.
Illnau and Effretikon were first mentioned in writing in AD 745. At first the name of the municipality was Illnau as that town used to be the political and economic center of the municipality. However, when the railway connection was built from Zurich to Winterthur in 1855, only the much smaller Effretikon was in a position to receive a railway station. Subsequently, the economic boom caused by the railway made Effretikon grow disproportionately until it had outgrown Illnau by the beginning of the 20th century. The name of the municipality was finally changed to Illnau-Effretikon in 1973.