Liquid biopsies can be obtained from blood, urine, cerebrospinal fluid, or even saliva, and they allow clinicians to assess multiple factors, transforming cancer management through a minimally invasive method and providing real-time molecular information about tumors. Circulating tumor DNA (ctDNA) contains tumor-specific genetic and epigenetic alterations, which allow early detection, disease assessment, and prediction of treatment responses. In early diagnosis, highly sensitive sequencing technologies can identify cancer-associated mutations and methylation patterns at very low concentrations, allowing earlier detection than traditional imaging or serum biomarkers. Detection of altered ctDNA now serves to distinguish true tumor signals from healthy genetic variants in early diagnostic settings. For disease assessment, alterations in ctDNA show a close correlation with tumor burden; thus, ctDNA can indicate disease progression weeks before any suspicion arises on imaging studies, while clearance of ctDNA after treatment correlates with improved outcomes. This ability positions ctDNA as a powerful tool for minimal residual disease surveillance and for determining the appropriate intensity of follow-up. As a predictive biomarker, liquid biopsies allow comprehensive genomic profiling without the need for repeated tissue biopsies and can reveal actionable mutations or resistance mechanisms that arise during therapy. Early shifts in ctDNA kinetics can also predict whether a patient responds to treatment or requires an alternative strategy. Liquid biopsies now hold an increasingly central role in personalized oncology, with significant contributions to early detection, treatment selection, and disease management.