Early-stage breast cancer (ESBC) is generally treatable, yet 15-25% of patients may experience a recurrence due to the presence of undetected residual disease that evades surgical procedures and conventional imaging techniques. Liquid biopsy the real-time examination of tumoral material shed into blood offers a non-invasive window into these invisible threats. Circulating-tumour DNA (ctDNA) leads the charge: ultrasensitive nextgeneration sequencing and methylome-based analytics now detect mutant alleles at parts-per-million levels, enabling minimal residual disease (MRD) identification within weeks of surgery or neoadjuvant therapy. In the ISPY 2 Signatera study, postoperative ctDNA clearance predicted pathologic complete response and long-term event-free survival. This predicts a future in which adjuvant treatment is escalated or de-escalated based on molecular, not radiologic, remission. Complementary analytes circulating tumour cells, exosomal RNA, tumoureducated platelets, and microRNA signatures add phenotypic breadth, capturing epithelial-to-mesenchymal transition, endocrine-resistance mutations, and immunogenic neoantigens even when ctDNA is scarce. Their integration into multi-omic panels raises sensitivity above 90 % for tumours smaller than 5 mm in pilot screening cohorts. Clinical translation is accelerating: GuardantReveal™ has secured FDA breakthrough designation for ctDNA-guided MRD surveillance. This chapter outlines the development of liquid biopsy in ESBC, covering topics such as ultra-deep sequencing and microfluidic CTC enrichment. It also presents evidence suggesting that molecular monitoring may be more effective than radiology over extended periods. It critically appraises current assays, dissects landmark clinical trials, highlights implementation hurdles, and sketches prospects in which bloodbased biomarkers guide screening, tailor adjuvant therapy, and ultimately close the relapse window for early-stage breast cancer.