Student peer effects are well documented. We know far less, however, about peer effects among teachers. We hypothesize that a relatively effective teacher may positively affect the performance of their peers, while a relatively ineffective teacher may negatively impact the performance of other teachers with whom they work 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. Interestingly, the incumbent teacher's students are not meaningfully disadvantaged by the entry of relatively ineffective peers. This finding implies that mixing teachers with diverse performance levels can be a strategy for increasing student achievement in the aggregate. These results are robust to several student sorting and teacher selection issues.