Alkaline phosphatase (ALP) is a hydrolase enzyme responsible for removing phosphate groups from molecules like nucleotides, proteins, alkaloids, etc. In humans, ALP is present in tissues throughout the entire body; but is mainly concentrated in liver, bile duct, kidney, bone and the placenta. Some important isoenzymes of ALP are intestinal ALP or ALPI, tissue-nonspecific ALP or ALPL, and placental ALP or ALPP or Regan isoenzyme. This chapter covers the method, principle, sample, reagent, procedure, calculation, normal range, and clinical significance of ALP.