Co-curricular and extracurricular involvement plays important roles in students' cognitive and effective development, educational effectiveness and satisfaction, and sense of belonging. Moreover, these practices can help equip a diverse population with the academic and professional skills necessary to succeed in the technological workforce. Yet undergraduate engineering students are consistently less likely than students in other majors to become involved in co-curricular and extracurricular activities. This study critically analyzes the body of literature focused on the extent to which involvement in out-of-class activities affects educational outcomes among both engineering and general undergraduate students. Employing a mixed research synthesis approach, this study evaluates 50 qualitative, quantitative, and mixed methods studies published between January 2000 and December 2014 using four steps. The findings suggest that out-of-class involvement supports undergraduate students' development of eight categories of outcomes and results in slight variations in outcome based on type of activity and major. This review synthesizes prior work, informs practice, and identifies opportunities for future research.