We use a systems approach to develop a predictive model that medical researchers can use to study and treat cancerous tumors. Our multiscale, cell-based model includes intracellular regulations, cellular level dynamics and intercellular interactions, and extracellular chemical dynamics. The intracellular level protein regulations and signaling pathways are described by Boolean networks. The cellular level growth and division dynamics, cellular adhesion and interaction with the extracellular matrix are described by a lattice Monte Carlo model. The extracellular dynamics of the chemicals follow a system of reactiondiffusion equations. All three levels of the model are integrated into a high-performance simulation tool. Our simulation results reproduce experimental data in both avascular tumors and tumor angiogenesis. This model will enable medical researchers to gain a deeper understanding of the cellular and molecular interactions associated with cancer progression and treatment.