Continuous glucose monitoring (CGM) systems have revolutionized diabetes management by providing real-time monitoring of glucose levels, reducing the risks of hypoglycemia and hyperglycemia. This paper offers a comprehensive review of both non-commercial and commercial CGM technologies, categorizing them based on optical, electrochemical, ultrasound, electromagnetic, and microwave principles. It highlights the various detection ranges, calibration requirements, and sensitivities to environmental factors of each system, emphasizing their advantages and limitations. Non-commercial systems, though innovative, still face challenges related to sensor lifespan, frequent calibration, and environmental interferences such as temperature and pH fluctuations. Commercial systems, particularly those like DexCom and Insulet's Omnipod provide more reliable performance within a physiological glucose range, offering seamless glucose management. Recent advancements, such as AI-driven algorithms and novel power sources, are discussed as potential solutions to the challenges of current CGM systems. The study concludes with an analysis of the critical limitations that must be addressed, including improving accuracy, extending sensor lifespan, and reducing calibration frequency, to further advance CGM technology and enhance diabetes care.