The term ‘liquid biopsy’ has become widely used by clinicians with the development of non-invasive diagnostic and monitoring techniques for malignancies. Liquid biopsy can provide genetic information for early diagnosis, risk stratification, treatment selection and postoperative follow-up. In the era of personalized medicine, liquid biopsy is an important research direction. In recent years, research on circulating tumour DNA (ctDNA) in hematological malignancies has also made great progress. This review provides an overview of the current understanding of circulating tumour DNA in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML). Additionally, recent advancements in the monitoring of minimal/measurable residual disease (MRD) through ctDNA are discussed.