TDM represents an important tool in the optimal use of selected psychiatric medications. Its role in therapy varies depending on the drug being prescribed. For some medications, such as lithium and tricyclic antidepressants, it should be a standard aspect of care. For other drugs, such as benzodiazepines, it is not helpful. Unfortunately, the use of TDM often follows an "all or none" pattern. Some clinicians rarely use it even for drugs like TCAs where it is critical to optimal care. Others use it excessively; repeating levels when it is not necessary or for drugs for which it is not necessary. It is hoped that this article has provided a basis for the thoughtful application of TDM in psychiatry as well as a review of the available data.