Philadelphia chromosome-associated leukemias are among the most well-understood human malignancies. The importance of BCR-ABL enzymatic activity in the proliferation of the leukemic clone has been confirmed by the high response rates of these leukemias to the ABL-selective tyrosine kinase inhibitor STI571, even in advanced disease phases, which are characterized by increased genetic heterogeneity. Disease relapse has been observed in a subset of patients who had initially responded to STI571. Evidence suggests that BCR-ABL activity is restored in the majority of these cases of acquired resistance. Molecular studies of resistant leukemia cells isolated from patients have implicated BCR-ABL kinase domain point mutation as the most common mechanism of resistance. Additionally, genomic amplification of the BCR-ABL gene can occasionally be detected. This review will highlight mechanisms of STI571 resistance in clinical samples as well as preclinical models.