This chapter describes tools and approaches for complex clinical situations that require specialized assessment. It reviews assessment principles regarding seven clinical presentations that require additional attention and careful consideration in treatment planning. Assessment of the relationship between the client's current symptoms and past traumatic events can be difficult, especially when the trauma occurred in the distant past, and the client may be unclear about the details and/or may not want to discuss them. The Stages of Change model of addiction helps in assessment and treatment planning with clients who use substances in a harmful way. There is a high incidence of co-occurring mental health and substance use disorders for adolescents and adults, and integrated treatment is generally more effective with clients who have symptoms of one or more mental health disorders as well as substance use disorders. Collaborative treatment planning with clients who have psychotic disorders helps to counteract the disempowerment that often accompanies psychosis.