Montelukast: Method development and Validation deals with the development of a new simple, rapid, specific, accurate and precise UV spectrophotometric method for Active Pharmaceutical Ingredient (API) and in pharmaceutical dosage form. The validation of the proposed method was carried out according to the I.C.H guidelines. Montelukast is a leukotriene receptor antagonist with anti-inflammatory and broncho-dilating activities. Having high affinity and selectivity for CysLT type 1 receptor it prevents any action at the receptor site from LTD4, LTE4 thus preventing allergic symptoms. Validation parameters covering all the qualitative and quantitative estimation of the drugs are mentioned along with the acceptance criteria as mentioned in the guidelines of the ICH. A simple and sensitive spectrophotometric method for montelukast as API and in pharmaceutical dosage form was developed. The new introduced method was found to be inexpensive, easy and reliable. The maximum absorbance for montelukast in methanol was found to be 270nm. The method was found to be progressive and behaves according to beers lamberts statute in concentration range of 5-45¿g/ml for both the drugs.