The Veterans Health Administration (VHA) is advocating that Veterans with serious mental illness (SMI) receive recovery-oriented, rehabilitation approaches that target real-world functioning. One such approach is Cognitive-Behavioral Social Skills Training (CBSST). Unlike traditional cognitive-behavioral therapy, CBSST is a more recovery-oriented psychosocial rehabilitation intervention that teaches Veterans with SMI to correct errors in thinking and build social skills. While effective, CBSST has only been tested when facilitated by masters- or doctoral-level therapists, which limits its use in VHA. However, our pilot data shows that Peer Specialists-individuals with SMI who are hired and trained to use their own recovery experience to assist others with SMI-can also provide CBSST (called CBSST-Peer). Stand-alone social skills training (SST) is also a recovery-oriented program that VHA is attempting to rollout nationwide for Veterans with SMI. A few Peer Specialists have been trained to co-lead SST with professionals. However, SST is not widely implemented because professionals are busy and Peer Specialist delivered SST has not been tested. This study will evaluate the effectiveness of Peer Specialist-delivered CBSST and SST, which would increase access Veterans with SMI have to effective treatment. Our aims are: Aim 1 (Effectiveness): To compare the impact of CBSST-Peer on outcomes in Veterans with SMI to Veterans receiving Peer Specialist-delivered SST and Peer Specialist-led manualized groups of equal duration and treatment as usual. We will also assess fidelity of SST and CBSST. Aim 2: (Helpfulness of CBSST/SST--Peer and implementation barriers and facilitators): To use focus groups with patients and interviews with Peer Specialists and other staff to assess perceptions of SST- and CBSST Peer and identify potential barriers and facilitators to future implementation. Methods: This is a randomized, Hybrid 1 trial involving 252 Veterans with SMI (n=126 each from Pittsburgh, San Diego) comparing 3 treatment arms: CBSST-Peer vs. SST-Peer vs. treatment as usual. Hybrid 1 trials test the effectiveness of an intervention and collect implementation data that could inform its future adoption. At each site, across 6 waves (a wave = 1 CBSST-Peer and 1 SST-Peer group), 2 Peer Specialists will co-lead 12 groups, each lasting 20 weeks. Peer Specialists will be trained and receive an hour of supervision weekly by the CBSST developers. Master trainers from the SST rollout will train and supervise Peer Specialists in each site. All three arms' sessions will be taped and 25% rated for fidelity on standardized measures. A survey battery that assesses functioning, quality of life, recovery, and symptoms will be administered to the Veterans in each wave at: baseline, mid-intervention (10 weeks), end-of-intervention (20 weeks), and follow-up (32 weeks, 3 months post intervention). We will examine all outcomes using Hierarchical Linear Models (HLM), with treatment condition included as a time-invariant covariate, and random intercepts for person and random slopes for time. Relevant covariates will include site, treatment attendance, symptom severity, service use, and demographic variables. We will evaluate the effect for treatment conditions (CBSST-Peer vs. SST-Peer vs. treatment as usual) in the expected direction and the time X group effect. Qualitative data on SST- and CBSST-Peer helpfulness and implementation factors will be collected from 8 focus groups, audio-recorded from a random sample of Veterans who participated in SST- and CBSST-Peer. Interviews will be conducted with participating Peer Specialists (n=approximately 8 per site), and key mental health staff (n=3-4 per site). The qualitative interviews and focus groups will be analyzed using rapid assessment, a team-based, iterative data collection and analysis approach providing data on the barriers and facilitators to future implementation of SST- and CBSST-Peer. Innovation: No study has tested peer-delivered SST or CBSST, or compared the two, in a rigorous trial. Significance/Expected Results: CBSST and SST are not widely available. If SST- or CBSST-Peer is effective, it could greatly increase the delivery of evidence-based services Veterans receive and enhance the services by VHA Peer Specialists.