Globally, cancer is one of the leading causes of death, with a significant proportion of cancer deaths due to delayed diagnosis. It is necessary to develop non-invasive, low-cost diagnostic techniques to ensure early and prompt cancer detection. Immunosensors have emerged as promising tools in the identification of cancer biomarkers for early cancer detection, as well as in the monitoring of cancer treatment. In this chapter, we describe immunosensors that have been fabricated for the detection of carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP), cancer antigen 125 (CA 125), cancer antigen 15-3 (CA 15-3), and prostate-specific antigen (PSA). We discuss current trends in cancer immunosensors for these specific cancer biomarkers, as well as their promises, challenges, and future directions.