Student peer effects are well documented; however, we know far less about peer effects among teachers. We hypothesize that a relatively effective teacher can positively affect the performance of his or her peers, whereas a relatively ineffective teacher may negatively affect the performance of other teachers with whom he or she works closely. Utilizing a decade of data on teacher transfers between schools that result in changes of peers when transfer teachers enter grade-level team in the new school, we find evidence of strong positive spillover effects associated with the introduction of peers who are more effective than the incumbent teacher himself or herself. However, the incumbent teacher's students are not meaningfully disadvantaged by the entry of relatively ineffective peers. This finding provides initial evidence that mixing teachers with diverse performance levels can increase student achievement in the aggregate. These results are robust to several student sorting and teacher selection issues.