This chapter explores the nature of clinical decision support systems (CDSS), describes some of the different types, and their value and application in clinical practice. There is consideration of some of the evaluation literature and the relevance of this implementation of decision support systems. CDSS have been broadly defined as any computer program that helps health professionals to make clinical decisions. Three types of expert system have been developed: rule based systems, probabilistic systems, and cognitive models. A system might scan test or laboratory results and by way of onscreen prompts or e-mail alert the clinician to important changes. A reminder system might alert a clinician that a patient is eligible for and has not yet received their winter flu vaccination. When a patient's case is complex, rare or the person making the diagnosis is simply inexperienced an expert system can be helpful.