Diagnosis, the key for successful management of the disease problem, refers to identification of the cause of a disease. The process of diagnosis requires clinical examination of the individual animal or the examination of herd. The clinical examination of the patient includes (i) history taking (anamnesis) - patient data, disease history, and management history, (ii) examination of patient including general inspection, inspection of body regions, physical examination of the patient, and laboratory investigation, (iii) examination of the environment - both indoor and outdoor. The general physical examination aims at determining abnormality of function. Special examination of a suspected organ or system(s) is carried out to ascertain the affected organ or system, and special examination of affected system helps to locate the lesion. Special examination of the lesion specifies the kind of lesion, and special examination of specimen determines the cause of lesion. The general physical and special examinations of a suspected organ or system are effectively completed by an experienced clinician with the help of routine diagnostic aids. The other steps however, require specific physical and laboratory examinations such as endoscopy, ECG, EEG, angiography, phonocardiography, ultrasonography, radiography, tomography, image intensification and scintigraphy.