The defining clinical strength of liquid biopsy lies in its ability to repeatedly monitor tumor dynamics over time. This enables assessment of clonal evolution, treatment response, recurrence, and resistance beyond what static tissue sampling can provide. ctDNA-based assays are currently the most widely applied in clinical practice, particularly for therapy monitoring and companion diagnostics. Applications are expanding toward early detection, MRD assessment, and surveillance during asymptomatic periods. However, blood-based biomarkers provide indirect tumor information and require rigorous validation of tumor specificity. Improving sensitivity alone is insufficient without biological and technical interpretive frameworks. Clinical performance must be evaluated according to the intended use of each assay. This chapter categorizes liquid biopsy applications based on clinical scenarios and decision-making contexts. It highlights where liquid biopsy can complement or potentially replace conventional diagnostics.