Particle formation processes by homogeneous and heterogeneous nucleation are becoming increasingly important for the preparation of functional materials. However, the complexity of the presently available nucleation theory, which considers the classical thermodynamics of the process, prevents its practical use. To overcome this, a simple and practical engineering model of particle formation by homogeneous and heterogeneous nucleation in the liquid phase has been proposed, which can predict the number concentration of nucleated particles as a function of the operating conditions. If the number concentration of nucleated particles can be predicted, their mean diameter can be determined from a mass balance. Experiments in which the precursor monomers were generated by several methods have successfully confirmed the predictions of the model. Furthermore, the original model developed for liquid-phase nucleation is extended to the case in which the precursor monomers are generated in a gas-phase system. A relationship between number concentration and mean volume diameter of nucleated aerosol particles and operating conditions is obtained considering the free molecular regime vapor condensation onto the critical nuclei, which is the main difference with the liquid-phase model, The predictions are in excellent agreement with the results obtained in gas-phase nucleation experiments. In the present paper, the simple model and the comparison of experimental results with the theoretical predictions are summarized.