Blood is a fluid thicker than water circulating uninterruptedly through the heart, arteries, capillaries and veins of the human body. It carries nutrients and oxygen to body tissues and takes away the waste products to the excretory organs of the body. Blood is made up of two major components, cellular and noncellular. About half of the blood volume is composed of blood cells, erythrocytes, leukocytes, and platelets. Noncellular component or blood fluid is known as plasma, which constitutes the other half of the blood. The normal values of different blood test and complete blood count have been described in a box format in this chapter.