A rehab unit is a specially equipped vehicle  that responds to major fires or emergencies at which the health and safety of the firefighters operating on the scene may be jeopardized by the nature or duration of the incident or the weather. The unit provides shelter, blankets, hot and cold beverages, and a place to cool off in the summer and warm up in the winter. Firefighters enter the unit for rest, hydration, food, and medical evaluation.

The National Fire Protection Association states in NFPA 1500, Standard on Fire Department Health and Safety Program, 1992 edition, "All supervisors shall maintain an awareness of the condition of members operating within their span of control and ensure that adequate steps are taken to provide for their safety and health." The standard further states that on-scene rehabilitation shall include at least basic life support.

Generally, a rehab unit will carry cots, blankets, and pillows for rest. It may also carry fluids for hydration; fans and misters for cooling off; awnings, tarps, and towels; and racks to hang equipment.

The safety and welfare of the firefighters on the scene should be the primary concern of the incident commander. In this regard, sufficient rest and recuperation are essential to prevent injuries, exhaustion, and overexposure.

The rehab unit provides an exhausted firefighter a place to rest for 20 to 30 minutes. For cases of overexposure to heat or the sun, the unit provides shelter, fluids, fans, misters, or just shade under an awning for cooling down. For overexposure to the cold, the unit provides shelter, blankets, and hot fluids. A rehab ambulance allows for medical evaluation and treatment.