The hours of service (HOS) are regulations issued by the Federal Motor Carrier Safety Administration (FMCSA) governing the working hours of anyone operating a commercial motor vehicle (CMV) in the United States for the purpose of "interstate commerce"— moving commercial goods from one U.S. state to another. This includes truck drivers and bus drivers who operate CMVs for motor carriers (their employers). These rules limit the number of daily and weekly hours spent driving and working, and regulate the minimum amount of time drivers must spend resting between driving shifts. For intrastate commerce, the respective state's regulations apply.

The FMCSA is a division of the United States Department of Transportation (DOT), which is generally responsible for enforcement of FMCSA regulations. The driver of a CMV is required to keep a record of working hours using a log book, outlining the total number of hours spent driving and resting, as well as the time at which the change of duty status occurred. In lieu of a log book, a motor carrier may keep track of a driver's hours using an electronic on-board recorder (EOBR), which automatically records the amount of time spent driving the vehicle.

The HOS's main purpose is to prevent accidents caused by driver fatigue. This is accomplished by limiting the number of driving hours per day, and the number of driving and working hours per week. Fatigue is also prevented by keeping drivers on a 21- to 24-hour schedule, maintaining a natural sleep/wake cycle (or circadian rhythm). Drivers are required to take a daily minimum period of rest, and are allowed longer "weekend" rest periods to combat cumulative fatigue effects that accrue on a weekly basis.

Enforcement of the HOS is generally handled by DOT officers of each state, and are sometimes checked when CMVs pass through weigh stations. Drivers found to be in violation of the HOS can be forced to stop driving for a certain period of time, which may negatively affect the motor carrier's safety rating. Requests to change the HOS are a source of contentious debate, and many surveys indicate some drivers get away with routinely violating the HOS. These facts have started another debate on whether motor carriers should be required to use EOBRs in their vehicles, instead of relying on paper-based log books.