A common feature of previous mathematical models is that they neglect the mechanic effects produced by the surrounding on tumour growth kinetics and dynamics. The stress field imposed on a tumour by the neighboring tissue (in vivo) or an external matrix (in vitro), as well as that generated internally by cells binds during expansion, is believed to play a pivotal role in tumour regulation. Several models exploited the concepts of oxygen diffusion, growth’s and death’s rate dependence on nutrient levels, porous media theory to predict migration of cells. The occurrence of necrosis is then made dependent on the attainment of a critical oxygen concentration threshold.