In this issue of BJPsych Advances Siddaway explores the challenges of assessing and treating post-traumatic stress disorder (PTSD) and complex PTSD. In this commentary I reflect on those challenges, not least of which is the need for a thorough understanding of different approaches to diagnoses. The very concept of diagnostic classification systems can be problematic, but when used sensitively they can aid communication, assessment and treatment. The relatively new diagnosis of complex PTSD may serve as a more accurate and more useful description of some psychological difficulties, leading to better treatment decisions. Good assessment, leading to accurate diagnosis, useful formulation and effective treatment takes time, and adequate resources should be allocated. Professionals can help patients to make well-informed choices about treatment options and they should offer evidence-based treatments without unnecessary delay.