Blood has a mysterious fascination for us from the dawn of time. It was considered the essence of life and was believed to cure diverse diseases and restore youth and vitality to the aged. Also, blood serves as an important diagnostic tool. Blood investigations are important and at times indispensable part of diagnostic process. Data from these investigations provide information that helps to make early and more definitive diagnosis that in turn lead to better therapeutic results. A blood test is a laboratory analysis performed on a blood sample to determine physiological and biochemical states such as disease, mineral content, drug effectiveness, and organ function. In our work we have classified the blood investigation under four headings, i.e. Hematology tests, Biochemistry tests, Microbiology tests and Serology tests with standard reference ranges, which can be very helpful for dental student as well as for practicing dentists for understanding and ordering the Blood Investigations.