The purpose of this paper is to highlight the important role that gender plays in organizing and affecting the quality of the classroom climate. We review research showing how students' peer relationships tend to be segregated by gender and discuss the consequences of children spending much of their time almost exclusively with same-gender peers, which perpetuates and exaggerates gender segregation. We suggest that a more beneficial pattern is enhancing students' exposure to other-gender peers -- that is, promoting gender integration. We describe the theories that support the benefits of integrating across social groups. Next, we review methods and results of strategies we have used to promote integration. Finally, we draw attention to the issues about gender integration that require further consideration and argue for more research that can help guide the development of effective strategies that build more inclusive environments, practices, and policies for today's children and youth. [The authors were also supported by funds from the T. Denny Sanford Foundation. This is the in press version of an article published in "Educational Psychologist."]