Methamphetamine (meth) is a major drug of abuse in California and several other states, particularly among criminal offender populations. Over the past decade, substance abuse treatment systems have had to adapt to and accommodate the increasing needs of meth users and, in California, deal with the impact of Proposition 36, which has resulted in a greater number of criminal offenders entering the treatment system. This study examines selected treatment performance and outcome indicators for California Proposition 36 offenders entering substance abuse treatment for meth use and compares their performance and outcomes to other subgroups of California treatment clients differentiated by whether or not they were admitted to treatment through Proposition 36 and whether or not their primary substance was meth. Significant improvements in all outcome domains were seen across the populations, and treatment performance and outcomes were not substantively inferior for the offender or meth-using groups.