Scale-up of universal social-emotional learning (SEL) programs has become a priority in schools to promote positive social outcomes for all students. Although studies have examined student outcomes associated with school-based SEL when extensive training and support are provided, research on teacher SEL implementation practices under typical conditions has received far less attention. As such, this study examined the first-year universal SEL implementation practices of 41 teachers across 13 schools in three states. School personnel made all decisions regarding approaches to training, support, and program implementation of a manualized program delivered at the classroom level to first and second grade students. Within this authentic context for implementation, variability was observed in selection of skills units, number of lessons taught, and adherence to lesson activities and scripts. Coding of teacher anecdotal reports revealed that perceived student needs, classroom context demands, and school-level factors informed teachers' implementation of the program. Impact and Implications Statement Few studies have examined implementation of universal SEL programs under typical conditions in elementary classrooms. The SSIS CIP is a CASEL-SELect program adopted in a number of U.S. schools. Results of the current study suggest that typical implementation of the program may deviate from the intended scope and sequence, which could undermine effectiveness. Thus, schools adopting this or other universal programs should consider how to proactively address competing factors such as curricular demands, time, and classroom needs. [This paper was published in "School Psychology" v37 n4 p285-297 2022 (EJ1372835).]