Research activities concerning the development and set up of a theoretical model for the analysis of spark-ignition two-stroke engines are reported. The engine system is identified by the definition of both zero-dimensional time-varying control volumes (i.e., cylinders or crankcases) and one-dimensional devices (i.e., intake or exhaust manifolds, transfer ducts, etc.). Fundamental processes such as combustion, fluid dynamics and scavenging, are modelled using up-to-date approaches. In particular, a fractal sub-model is adopted for the evaluation of flame area and burning rate; a high resolution upwind TVD scheme is utilized for the prediction of wave propagation within ducts.The overall prediction level is estimated through the comparison with experimental data measured on a small-size engine under both motored and firing conditions. The proposed model achieves a satisfactory compromise between accuracy and computational costs, if compared with more advanced CFD calculations, and allows the design and the analysis of a large number of different engine configurations to be performed.