M-66 is a north–south state trunkline highway on the Lower Peninsula (LP) of the US state of Michigan. It runs from the Indiana state line in the south to Charlevoix in the north. M-66 is the only state highway to traverse almost the entire north–south distance of the LP. It starts as a continuation of State Road 9 (SR 9) which provides access to the Indiana Toll Road. The total length is 272.898 miles (439.187 km), which includes 3.374 miles (5.430 km) of freeway between Interstate 94 (I-94) and downtown Battle Creek designated as I-194. One section of the highway is an expressway, a type of divided limited access highway, while the section along I-194 is a full freeway, otherwise M-66 is a two-lane rural highway. Two sections are listed on the National Highway System.