The mathematical models presented and analysed in some detail in this chapter are of two types: one approach applies a continuum, deterministic approach (based on systems of reaction-diffusion-taxis equations), the other approach applies a discrete, quasi-stochastic approach (based on a biased random-walk model). The basic invasion model takes into account the dynamics of cancer cells, extracellular-matrix and matrix degrading enzymes. Then the role of the urokinase plasminogen activator system (uPA) is discussed.