A retrospective review of the most important researches made regarding non-invasive blood glucose monitoring is presented, and bases for further investigation in this field are offered. The problem of diabetes disease on its two known types as well as its consequences and its management is discussed, and the need for this kind of glucose monitoring to avoid the painful procedures is shown. Fundamentals of microwave technology concerning relative permittivity measurement and tissue characterization are provided to understand the operating principles of non-invasively measuring, and theory review on this concern is presented. Variations of the relative permittivity in some zones of the body as the blood glucose level changes are studied to assess the feasibility of this kind of glucose monitoring. Literature references of electrical properties of biological tissues are studied, and the most relevant researches made regarding non-invasive blood glucose monitoring are discussed, focusing on the systems based on microwave resonators.