Laboratory tests are frequently used in the investigation and monitoring of patients with cardiac disease and failure. These can broadly be divided into general laboratory tests that may be affected by the presence of heart disease or failure, and specific laboratory tests that provide information directly pertinent to the heart. Broadly, there are three motives for the performance of general tests and particularly biochemical, haematological and urine analyses in patients undergoing investigation for cardiovascular disease. The first is to allow the clinician to rule in or out significant concurrent disease that might be responsible for the clinical signs observed; thus the tests aid in the differential diagnosis of the case. The second is to assist in the identification of underlying systemic diseases that may contribute to the development or exacerbation of the cardiovascular disease identified; examples of this would be in helping to rule in or out renal disease in elderly cats with evidence of cardiovascular abnormalities, or endocrine disease in older patients with evidence of cardiovascular dysfunction. Finally, and perhaps most importantly, the performance of biochemical analysis allows evaluation of the impact of cardiovascular disease and its treatment on the function of other organ systems. The chapter discusses General tests; and Specific tests.