According to the World Health Organization (WHO), cancer is one of the top causes of mortality worldwide, accounting for about one in six fatalities and about 10 million deaths in 2020. Since many malignancies have far higher survival rates when detected early, early identification is essential. However, the invasiveness, cost, and time required for established diagnostic techniques like imaging and biopsies limit their applicability for routine use or mass screening. As a result, biosensors have become revolutionary instruments that provide quick, affordable, and humane cancer diagnosis methods. Biosensors are analytical tools that combine transducers and biorecognition components, including cells, enzymes, antibodies, or nucleic acids, to transform biological interactions into quantifiable signals. At the molecular and cellular levels, they can identify cancer biomarkers, including proteins, exosomes, RNA, and circulating tumor DNA (ctDNA). The global biosensor market is expected to grow from its estimated $25 billion in 2021 to over $40 billion by 2028, demonstrating the growing importance of these instruments in medical diagnostics. To increase detection accuracy, biosensors frequently use labeling strategies like electrochemical markers or fluorescence. Impedance-based biosensors and surface plasmon resonance (SPR) are examples of label-free technologies that provide direct detection without modifying the material. By detecting distinct molecular profiles or mutations, these developments allow for early discovery, tracking the course of the disease, and individualized treatment. Biosensors are effective, noninvasive devices that identify biomarkers in biological fluids such as blood, saliva, or urine. This allows for regular testing and minimizes patient pain. They are perfect for point-of-care diagnostics because of their portability, which expands their use outside of conventional labs. Their performance is further improved by advancements in artificial intelligence, microfluidics, and nanotechnology, which enable real-time analysis and increased sensitivity. Biosensors are transforming early detection and tailored medication by offering precise, reasonably priced, and easily accessible cancer diagnostics. They have the potential to greatly transform oncology care worldwide and increase survival rates. This chapter examines the use of several biosensor variations in the fight against cancer.