Liquid biopsy using circulating tumor DNA (ctDNA) has been reported to be less invasive and effective for comprehensive genetic analysis of heterogeneous solid tumors, including decision-making for therapeutic strategies, predicting recurrence, and detecting genetic factors related to treatment resistance in various types of cancers. Breast cancer, colorectal cancer, and lung cancer are among the most prevalent malignancies worldwide, and clinical studies of liquid biopsy for these cancers are ongoing. Liquid biopsy has been used as a companion diagnostic tool in clinical settings, and research findings have accumulated, especially in cases of colorectal cancer after curative resection and non-small cell lung cancer (NSCLC) after curative chemoradiotherapy, in which ctDNA detection helps predict eligibility for adjuvant chemotherapy. Liquid biopsy using ctDNA shows promise across a wide range of cancer types, including breast cancer, and its clinical applications are expected to expand further through ongoing research. In this article, studies on liquid biopsy in breast cancer, colorectal cancer, and NSCLC are compared focusing on ctDNA.