This chapter discusses serum urea estimation, where the normal blood urea level ranges between fifteen and forty milligrams per decilitre, which represents a balance between its production and excretion. The concentration of blood urea rises if excretion is impaired and this frequently occurs in renal diseases. Apart from renal disease, other factors also do elevate blood urea. The concentration of urea in blood as such is dependent on diet. The greater the protein intake, the greater is the urea formation. Estimation of blood creatinine is preferable to urea estimation and therefore a good index of the renal status. Urea reacts directly with diacetyl monoxime under strongly acidic conditions to give a yellow condensation product. The reaction of urea with diacetyl monoxime is intensified by the presence of ferric ions and thiosemicarbazide and the resulting red colored complex formed is a measure of the Urea concentration.