Four research reports in this issue highlight some of the many ways in which co-occurring substance use disorders complicate treatment and worsen outcomes for people with mental illness. Robin E. Clark, Ph.D., and colleagues analyzed Medicaid claims data from six states for nearly 150,000 adult enrollees with mental disorders. For the large subgroup with co-occurring substance use disorders (29%), expenditures for medical care for physical health problems were substantially higher in five states. In fact, the cost of providing such care was greater than the direct cost of mental health and addictions treatment for this subgroup (page 35). Jeffrey D. Baxter, M.D., and colleagues used two measures to examine the quality of asthma care for more than 19,000 adult Medicaid enrollees who had behavioral health disorders and comorbid asthma. Those with substance use disorders were particularly likely to receive poor-quality asthma care (page 43). In a study of nearly 3,000 veterans with bipolar disorder, Jennifer C. Hoblyn, M.D., M.P.H., and colleagues found that 20% had a psychiatric hospitalization during the study year. Three variables conferred a 100% risk of hospitalization: all veterans in this group who had both an alcohol use disorder and polysubstance dependence and who were separated from their spouse or partner were hospitalized (page 50). Marlys Staudt, Ph.D., M.S.W., and Donna Cherry, Ph.D., M.S.W., used data from a national study of the child welfare system to investigate whether parents with mental or substance use disorders were offered and received appropriate services. Parents in both groups used services at a higher rate than in the general population, indicating that child welfare caseworkers facilitated treatment; however, those with substance use problems were significantly less likely to be offered treatment (page 56).