Neuropsychological testing--medical imaging of the brain structure and function--allows the expert to inform the court on the brain structure and function of the forensic examinee. Supported by extensive clinical use, neuropsychological testing and structural imaging in the form of computerized tomography and structural magnetic resonance imaging have achieved general acceptance in court. However, functional imaging such as functional MRI and nuclear medicine techniques, such as positron emission tomography (PET), have faced more admissibility challenges. While functional imaging is becoming an increasingly important tool in assessing neuropsychiatric illness, we surmise that evidentiary challenges are largely related to the phase of trial in which the nuclear study is offered as evidence. This article will review the basic science of functional nuclear imaging including PET and single photon emission computed tomography. We will then review cases where admissibility of these techniques has been challenged and consider whether and how nuclear brain imaging can influence the outcome of the trial.