This paper discusses the limits of expert opinion on posttraumatic stress disorder (PTSD) in personal injury claims. The construct of PTSD is hampered by several empirical limitations. Multiple reliable measures of PTSD exist, but have not been evaluated sufficiently within litigating samples and are infrequently used by forensic assessors. Common methods for trauma screening appear insensitive. Opinions about causation of PTSD and disability are complicated by retrospective memory biases, as well as the failure of most anxiety disorders to be detected within primary medical care. PTSD appears to have a steep spontaneous remission curve during the first year, but at least 10% of trauma-exposed people suffer chronic distress. Little is known about the course beyond 1 year. Efficacious psychological treatments have been developed for PTSD, but are not in common use limiting claimants' access to rehabilitative treatments. Research on functional disability associated with PTSD is in its infancy, but it seems likely that PTSD will account for only a part of the variance in work disability. We provide suggestions for improving forensic practice, advising the courts about the limitations of forensic opinions, and necessary research.