Animal models of BCR–ABL+ leukemias have provided important new knowledge about the molecular pathophysiology of these diseases, and answered questions that are difficult or impossible to address using BCR–ABL-expressing cell lines or primary Ph+ leukemia samples from patients. The power of mouse models lies in their ability to recapitulate precisely the phenotypes of BCR–ABL+ leukemias in vivo, but this comes at the price of significant complexity. Here I review recent studies of leukemias induced in mice by BCR–ABL with an emphasis on the intricate nature of these diseases and the need for careful pathological and molecular analysis.