In 2010, the Department of Veterans Affairs (VA) and Department of Defense (DOD) jointly published the revised VA/DOD Clinical Practice Guideline for Management of Posttraumatic Stress. The Clinical Practice Guideline (CPG) provides evidence-based recommendations for diagnosing and treating a spectrum of stress-related disorders. Included in the CPG were recommendations for assessing posttraumatic stress disorder (PTSD) and other stress disorder-related functional impairment. This article complements those CPG recommendations by providing information that may further guide clinicians in the assessment of functional impairment related to PTSD and other stress-related disorders. We briefly review some of the empirical literature on the association between PTSD and functional impairment and some of the more frequently used methods and measures for assessing functional impairment and introduce a new measure currently being developed by our group. We suggest that information obtained via patient self-report and/or clinician rating be supplemented whenever possible with collateral data from friends, family members, coworkers, or supervisors to provide a complete picture of current and premorbid functional status. Finally, we explore several important issues that we encourage clinicians to keep in mind when assessing functional impairment among Veterans and Active Duty servicemembers.