Posttraumatic stress disorder (PTSD) is a disabling psychiatric disorder that affects 20%-30% of U.S. Veterans. PTSD is strongly associated with increased risk for substance abuse comorbidity, including cannabis use disorder; however, multiple states now include PTSD as a condition for which patients can be legally prescribed medical marijuana, despite the fact that there has not been a single large-scale randomized clinical trial demonstrating the efficacy of cannabis to treat PTSD to date. The overall objective of the current proposal is to prospectively study the impact of reduced cannabis use on psychosocial functioning among Veterans with PTSD. To do so, we will first use ecological momentary assessment (EMA) methods to evaluate the relationship between cannabis use and daily functioning among Veterans with PTSD. Next, we will use mobile contingency management (CM) and EMA to assess the impact of reduced cannabis use on daily functioning among Veterans with PTSD who are heavy cannabis users. Our central hypothesis is that reductions in cannabis use will lead to positive changes in the functional outcomes of Veterans. The rationale for this research is that it will provide the first and only real-time data concerning the impact of reduced cannabis use on daily functioning among Veterans with PTSD. As a result, this innovative and timely project has the potential to significantly advance VHA healthcare and will directly inform the ongoing national debate concerning the impact of cannabis use on the long-term functional recovery of Veterans with PTSD.