Optical spectroscopy (OS) is a tissue-sensing technique that could enhance cancer diagnosis and treatment in the near future. With OS, tissue is illuminated with a selected light spectrum. Different tissue types can be distinguished from each other based on specific changes in the reflected light spectrum that are a result of differences on a molecular level between compared tissues. Therefore, OS has the potential to become an important optical tool for cancer diagnosis and treatment monitoring. In recent years, significant progress has been made in the discriminating abilities of OS techniques between normal and cancer tissues of multiple human tissue types. This article provides an overview of the advances made with diffuse reflectance, fluorescence and Raman spectroscopy techniques in the field of clinical oncology, and focuses on the different clinical applications that OS could enhance.