Heterogeneous materials present a mechanical response strongly dependent on the static and kinematic phenomena occurring in the constituents and at their joints. In order to analyze this kind of materials it is a common practice to distinguish a macroscopic length scale of interest from a mesoscopic one, where the mesoscopic length scale is of the order of the typical dimensions of the constituents. At the mesoscopic level the interaction between the units is simulated by mean of apposite mechanical devices. Among these devices is popular the zero thickness interface model where contact tractions and displacement discontinuities are the primary static and kinematic variables respectively. However, in heterogeneous materials the response also depends on joint internal stresses as much as on contact stresses. The introduction of internal stresses brings to the interphase model or an enhancement of the classical zero-thickness interface. With the term 'interphase' we shall mean a layer separated by two physical interfaces from the bulk material or a multilayer structure with varying properties and several interfaces. Different failure conditions can be introduced for the physical interfaces and for the joint material. The interphase model has been implemented in an open-source research-oriented finite element analysis program for 2D applications. Numerical simulations are provided to show the main features of the model. Copyright © (2013) by International Conference on Fracture.