Forensic mental health assessment (FMHA) comprises evaluations conducted by mental health professionals to help inform legal decision-making. This article reviews the topic of FMHA in four sections. Section 1 provides relevant definitions and context. Section 2 describes foundational principles of FMHA – broad, general principles that apply to all different kinds of forensic mental health assessments, whether conducted in criminal or civil contexts. Section 3 provides more detail on the most common kinds of legal questions informed by FMHA. Finally, Section 4 outlines areas that provide support for FMHA: law, science, ethics, and practice. It also describes professional organizations associated with FMHA.