The study explored mental health service use patterns and barriers to care among individuals with comorbid mental and substance use disorders.Using data from the National Survey on Drug Use and Health (2005-2010) for 18,972 adults with past-year major depressive episodes, the study compared mental health service use and perceived barriers to care among participants with and without co-occurring alcohol dependence, nonalcohol drug dependence, and both alcohol and drug dependence.Compared with participants without comorbid substance dependence, participants with alcohol dependence or both alcohol and nonalcohol drug dependence used more mental health services of all types, and participants with only comorbid alcohol dependence used more medication treatments. Participants with comorbid substance dependence were significantly more likely than those without comorbid substance dependence to report unmet mental health treatment need. However, barriers to mental health care were remarkably similar across groups, with financial barriers being the most common in all groups.Participants with major depression comorbid with substance dependence used more mental health services but also perceived more unmet need for such care than individuals without such comorbidity. However, barriers to mental health care were similar across groups with and without comorbidity. Policies aimed at expanding insurance coverage and mental health parity would likely benefit individuals with major depression and substance dependence comorbidity even more than those without such comorbidity.