This article's objectives are to guide psychiatrists caring for the medically ill in deciding when to request an electroencephalogram (EEG), in choosing what to do to maximize the possibility of eliciting positive findings, and in determining how to apply information in the EEG report. Although major advances in imaging techniques have eclipsed the EEG's importance as a tool for studying brain structure in recent decades, it continues to have a significant role in clarifying psychiatric presentations in the medical-surgical setting. EEG remains the pre-eminent cost-effective and readily available method for identifying brain dysfunctions such as delirium and seizure disorders, which often masquerade as purely psychiatric conditions whose organic bases go undetected. This article emphasizes the EEG's role as a correlational rather than a screening tool. A series of case reports illustrate how the EEG can be used to tease apart a diagnostic differential that can include entities as diverse as acute brain dysfunction, longstanding personality structure, and intermittent seizure discharge. The thoughtful integration of EEG data with material from clinical history and examination is fundamental to the EEG remaining a useful component in a psychiatric diagnostic armamentarium.