This book contains both review and synthesis of new pyrazolo[34-d]pyrimidine derivatives as cycline dependent kinase inhibitors (CDKIs). Moreover cytotoxic activity for the newly synthesized compounds were evaluated specially on breast cancer cell line (MCF-7). The results obtained were compared with methotrexate as a reference drug. Docking study and molecular modeling were also done using MOE program. I hope this work to be helpful and a starting point for much more new compounds with better activity to help in treatment of such a big terrible disease like cancer. Author: Phoebe F. Lamie