Diabetes has recently become the leading cause of death worldwide. So far, there is no effective treatment to cure or prevent diabetes. Still, reasonable blood control through glucose monitoring can improve treatment efficiency, relieve symptoms, and reduce the complications of the disease. However, conventional glucose detection is based on the finger-prick measurement, which may bring discomfort and pain to patients. Electrochemical-based continuous glucose monitoring (CGM) devices have been commercialized and appreciated by patients. However, those sensors still have high costs, short lifetime, and frequent calibration via finger-prick measurement. In recent studies, as a promising method for glucose quantification, optical glucose sensing technology has been considered a potential alternative to electrochemical CGM sensors. A commercial CGM sensor based on fluorescence sensing has been developed and can be worn for a longer period before a replacement. This paper aims to review optical methods for CGM, including near-infrared (NIR) spectroscopy, mid-infrared (MIR) spectroscopy, Raman spectroscopy, photoacoustic (PA) spectroscopy, fluorescence technology, optical coherence tomography (OCT), holographic technology, and hydrogel sensing technology in aspects of principles, current research, and limitations. Discussions and comparisons in these different optical glucose sensing technologies are also conducted. Moreover, the review discusses the future prospects for optical glucose sensing methods and concludes that further optical CGM research should focus on the improvement of data processing methods.