Tumor genomic sequencing has become part of routine oncology practice in many tumor types, in order to identify potentially targetable mutations and to personalize cancer care. Plasma genotyping via circulating tumor DNA analysis is a noninvasive and rapid alternative method of detecting and monitoring genomic alterations throughout the course of disease. Multiple assays have been developed to date, each with different test characteristics and degrees of clinical validation. Here we review the clinical data supporting these different plasma genotyping methodologies, and present a practical approach to the interpretation of the results of these tests. While the clinical application of plasma genotyping has been most extensively validated in the metastatic setting-for the detection of targetable alterations at the time of initial diagnosis or disease progression-this technology holds significant promise across many tumor types and stages of disease. We will also review emerging applications of plasma genotyping that are currently under clinical investigation.