The function of the kidneys is to regulate the volume and composition of extracellular fluid. This is achieved by the initial formation of an ultrafiltrate of plasma by the passage of solutes, small proteins and other non-scellular constituents of the blood across the glomerular filtration barrier. The chapter deals with glomerular filtration, laboratory abnormalities in patients with kidney disease, staging chronic kidney disease, acute kidney injury, glomerular barrier function and tubular function. This chapter includes case examples.