The present invention is directed to novel 2-amino-3,4-dihydro-pyrido[3,4-d]pyrimidine derivatives, pharmaceutical compositions containing them and their use in the treatment of Alzheimer's disease (AD) and related disorders. The compounds of the invention are inhibitors of .beta.-secretase, also known as .beta.-site cleaving enzyme and BACE, BACE1, Asp2 and memapsin2.