The current study makes several contributions to the extant literature on the relationship between immigration and neighborhood crime. I review classical and contemporary theories and argue that these theories make contradictory predictions regarding the moderating effects of ethnic heterogeneity on the immigration and crime relationship. Previous immigration and crime studies cannot help adjudicate between these positions because they have only considered diversity as a mediator or a control variable. I use multiple measures of diversity to conduct the first comprehensive study of the moderating effects of ethnic heterogeneity on the immigration and violent crime relationship at the neighborhood level. The results indicate that greater diversity strengthens the protective effect of immigrant residential concentration. These findings contradict the assumptions of classical theories and support the more recent immigration and crime perspectives.