Crystalline forms of (S) 1 (4 (5 cyclopropyl 1H pyrazol 3 ylamino)pyrrolo[1 2 f][1 2 4]triazin 2 yl) N (6 fluoropyridin 3 yl) 2 methylpyrrolidine 2 carboxamide of the formula (I)are provided. Also provided is a pharmaceutical composition comprising one or more crystalline forms of (3R 4R) 4 amino 1 [[4 [(3 methoxyphenyl)amino]pyrrolo[2 1 f][1 2 4]triazin 5 yl]methyl]piperidin 3 ol as well as a method of using one or more crystalline forms of (3R 4R) 4 amino 1 [[4 [(3 methoxyphenyl)amino]pyrrolo[2 1 f][1 2 4]triazin 5 yl]methyl]piperidin 3 ol in the treatment of cancer and other proliferative diseases.