A partial differential equation (PDE) is an equation that contains partial derivatives in one or more variable(s). For instance, a PDE can be used to describe changes in time and space. Therefore, systems of PDEs can be considered as a natural extension of ODEs (ordinary differential equations). PDEs can be applied to a wide range of biological processes such as spatial signal transduction and cell-cell communication, mechanical forces in plant or fungal cell walls as well as membrane bending and stretching. While linear PDEs can often be invesigated analytically, solutions to non-linear PDE systems are usually obtained numerically by using the finite element method (FEM).