Cancer is a life-threatening disease and among the leading causes of morbidity and mortality worldwide. There is a lot of uncertainty when it comes to the patients’ prognosis based on the conventional available diagnostic and therapeutic approaches. The most commonly used diagnostic method for cancer is tissue biopsy, but the process is lengthy and that can be damaging to the patient. Early diagnosis of cancer can be significantly improved by the use of a biomarker-based cancer diagnostic technology which in turn can result in successful treatment. The detection of biomarkers has raised interest recently due to the need for diagnostics and personalized medicine. Biosensors are important in biomarker detection and allow for accurate, rapid, and convenient testing at the point of care (POC) irrespective of the geographical location of the patient. There are several types of biosensors that are currently available, and in this book chapter, we focus on optical-based biosensors which are the most distinguished in biosensing owed to their biosensing capabilities, promptness, and easiness in detecting signals.