Clinical pathology is a branch of pathology, which mainly support the diagnosis of a disease using laboratory test of blood and body fluids. While the conduct of all the tests, analysis, and reporting is the responsibility of the pathologist, this clinical branch contributes significantly to the interpretation of the investigations, arriving at a diagnosis, evaluating the progress of the disease, thus contributing to the management of the patient. Even though the disease may be local, we are aware that there is a strong communication between various systems of the body; hence, the evaluation of the blood, urine, and other body fluids would reflect the ongoing pathologic conditions, stage of the disease. Hence, the routine investigations form the part of the workup of patients at early stage of the disease and the clinicians invariably top it up with advanced investigations based on the progress of the disease. Some of the investigations contribute to pathognomonic diagnostic criteria of a disease: for example, identification of a malarial parasite in blood, blast cells in leukemia, or identification of sugar in urine in diabetes, while other tests aid in the ruling in or ruling out the provisional diagnosis made by the clinician who is examining the patient: for example, high WBC count in infections, low Hb in anemia, etc., This chapter provides a brief overview of the routine investigations as well as some of the advanced investigations commonly used in ophthalmic pathology laboratories. Hence, the intention of including this chapter is to update knowledge of clinicians, as well as the principles, limitations, and interpretation of commonly prescribed routine investigations. In this chapter, we discuss the basics of hematology, body fluids, and common autoimmune markers, which will help us in understanding the ophthalmic pathology.