Intratumor heterogeneity within individual cancer tissues underlies the numerous phenotypes of cancer. Tumor subclones ultimately affect therapeutic outcomes due to their distinct molecular features. Drug-resistant subclones are present at a low frequency in tissues at the time of biopsy, but can also arise as a result of acquired somatic mutations. A number of different approaches have been utilized to understand the nature of intratumor heterogeneity. Clonal analysis using whole exome or genome sequencing data can help monitor subclones in the context of tumor progression. Multiregional biopsies permit the molecular characterization of subclones within tumors. Deep sequencing has also provided researchers with the ability to measure the low allele fraction variant within a small number of cells. Ultimately, single-cell sequencing will enable the identification of every minor population within a tumor microenvironment. In the clinical context, the ability to identify and monitor the subclonal architecture of a tumor is valuable for the development of precise cancer therapeutic methods.