The prevalence of an internal tandem duplication (ITD) of the juxtamembrane domain-coding sequence and a missense mutation of D835 within the kinase domain of the FLT3 gene is 15–35% and 5–10% of adults with acute myeloid leukemia (AML), respectively. In addition, point mutations, deletions, and insertions have been found in the juxtamembrane domain and in the other codons within the kinase domain, though these are less common. Several large-scale studies in well-documented patients published to date have demonstrated that FLT3 mutations are strongly associated with a poor prognosis and a high leukemia cell count in patients with AML, suggesting that FLT3 mutations are involved in disease progression. Because the detection of FLT3 mutations is fast, easy, and inexpensive, mutation analysis should be performed as a routine test. This chapter describes methods for detecting ITD and D835 mutations in the FLT3 gene.