Abstract: The role of immune checkpoint inhibitors (ICIs) against solid tumors has been gradually recognized, and ICIs are being clinically applied. However, spatial and temporal heterogeneities of tumors have hindered the clinical use of biomarkers to evaluate the efficacy of ICIs. Real-time dynamic monitoring of therapeutic effects is necessary to understand the gene mutation, metastasis, tumor recurrence, and treatment response. Circulating tumor cells (CTCs) and circulating tumor DNA (ctDNA) analyzed by liquid biopsy are expected to become candidate markers for predicting the efficacy of immunotherapy because they are not affected by tumor heterogeneity. This article reviews the application of CTCs and ctDNA in tumor immunotherapy to provide theoretical support for clinicians to choose reasonable and effective predictive biomarker indicators.