Hematology is one of the essential tools in the diagnosis of diseases in fish and in understanding physiological and pathological conditions. This chapter presents an overview of hematological techniques, highlighting their importance in aquaculture and in the management of fish health. The introductory chapter establishes the importance of blood analysis in detecting diseases, monitoring stress, and evaluating the overall health of the fish. Understanding factors that affect fish hematology, such as stress, anesthesia, sampling methods, use of anticoagulants, and blood storage, is reviewed so that no diagnostic error can be made. The constituent components of the blood, which include plasma, serum cellular elements, erythrocytes, and leukocytes, are elaborated on in detail. Such structures and functions of blood in fish are underlined. The procedures related to blood sampling and techniques for the collection and manipulation of blood are described with “practical tips” on how to handle the blood sample to minimize stress and contamination. In this chapter, the following methods for blood analysis are mentioned—RBC parameters, Ht, Hb, and derived erythrocyte indices like MCV, MCH, and MCHC. The discussion also discusses erythrocyte morphology as a diagnostic indicator. WBC parameters are further discussed in this chapter, mainly on the DLC and types of leukocytes that play an important part in immunity. The conclusion points out the importance of hematological techniques in the diagnostics of fish health, with an emphasis that standard techniques should be used to increase precision and reliability in the practice of aquaculture.