Although it is not uncommon to be taught that a MH diagnostic assessment cannot begin until a client is substance free for 30 days or more, current practice recognizes that this approach creates a significant barrier to timely engagement and intervention. It is now recognized that it is important to use clinical best practice assessment processes to begin the diagnostic assessment immediately, even if the patient is currently or recently using substances, not just because of administrative and billing rules, but more importantly, so that the client can more quickly receive appropriate care, including medication. The fundamental rule is that diagnostic assessment is primarily based on getting a good history, regardless of how the person presents symptomatically. A good history will provide a diagnosis almost all the time, even though the person’s substance use may influence the immediate symptoms. Putting it simply, if the person has a long history of schizophrenia, bipolar disorder, PTSD, etc., that illness does not go away because they are binging on alcohol. History should be confirmed by consulting previous records and by talking with collaterals, and initial diagnoses may change as more information is gathered, but it is helpful to have a starting place. Further, if you want to know what the person looks like when they have been abstinent for a period of time, use your assessment to carefully identify and assess previous periods of abstinence, including in a controlled environment, for their baseline mental status at that time.