Biosensors are sensor systems that are combined with biological systems. They consist of two main parts; one of them is biological recognition element and the other one is physicochemical transducer. Biological recognition element can be biocatalysts (enzyme, microorganism, tissue materials) or bioligands (antibodies, nucleic acids, lectins) that interact selectively with the target analyte. Transducer converts the physical response resulting from the interaction between the target analyte and biological recognition element into a measurable signal. Biosensors which provide monitoring biological and synthetic processes can be used besides medical measuring and analyzing, for environmental observations and controls, defense industry, agriculture, food and pharmaceutical industry. Thus, they have an important role in daily life. In this review, characteristics of biosensors that are used in medical measuring and analyzing have been determined and classifications of biosensors according different features that are found in literature are presented. [Archives Medical Review Journal 2013; 22(4.000): 499-513]