In firefighting, the policy of two-in, two-out refers to United States Occupational Safety and Health Administration (OSHA) policy 29 CFR 1910.134(g)(4)(i). The respiratory protection standard requires that workers engaged in fighting interior structural fires work in a buddy system; at least two workers must enter the building together, so that they can monitor each other's whereabouts as well as the work environment. There must also be at least two standby personnel outside the fire area prepared to rescue the inside firefighters should the need arise. One of these outside firefighters must actively monitor the status of the inside fighters but the second outside firefighter may perform a variety of other duties, such as pump operations, incident commander or outside hose line operation. There are no provisions in the standard to waive the requirements for either the "two-inside firefighters" or the "two-outside firefighters", although the circumstances under which this provision applies are more limited than generally understood.

The standard does not require the "two-in/two-out" provision if the fire is still in the incipient stage and it does not prohibit firefighters from fighting the fire from outside before sufficient personnel have arrived. It also does not prohibit firefighters from entering a burning structure to perform rescue operations (Search) when there is a reasonable belief that victims may be inside. It is only when firefighters are engaged in the interior attack of an interior structural firefighting that the "two-in/two-out" requirement applies. It is the incident commander's responsibility to judge whether a fire is an interior structural fire and how it will be attacked.