Liquid biopsy techniques using cell-free DNA (cfDNA) play an increasingly important role in the characterization and surveillance of solid tumors. For blood cancers, molecular response assessment techniques using circulating malignant cells or bone marrow aspirates are well established in clinical care. However, cfDNA has an emerging role in hematology as well, with the opportunity for disease assessment and quantification independent of circulating disease burden or invasive biopsies. In this review, we discuss key technologies and clinical data for the utilization of cfDNA in lymphomas, myeloma, and leukemias.