The success or otherwise of a surgical procedure can depend largely on good planning, awareness of potential risk factors and selection of the appropriate procedure. It is therefore critical that thorough preoperative patient evaluation is performed in every case, to ensure that surgical risk is minimized and that the chance of a successful outcome is maximized. The chapter covers Taking a clinical history; Clinical examination; The critical patient; Clinical pathology; Diagnostic imaging; Other investigations; Establishing surgical risk; and Communication with the client. Also included are guides to performing the following techniques: Needle thoracocentesis; Abdominocentesis; and Diagnostic peritoneal lavage.