In recent years microturbines and, more generally, small radial turbomachinery have been of great interest to the power industry due to their possible integration with advanced energy systems involving very high-tech components, such as high temperature fuel cells, high temperature ceramic heat exchangers and air saturators, which are still under development. Even if microturbine technology is already commercially available, when such an engine is inserted into a more complex plant there are at least two difficulties to be faced: firstly, guaranteeing the safe operation of the microturbine at all the operating points, including start-up and shut-down; and secondly, ensuring the proper feeding conditions to the main components of the cycle. In fact, additional components bring new variables to be monitored and require additional control devices and control systems. As a result, a flexible and modular simulation tool is extremely useful for the actual development of new cycles, where attention is mainly paid to the interaction of quasi-conventional turbomachinery with advanced components integrated in the same cycle. The TRANSEO code is a MATLAB-based modular tool for the transient performance simulation of conventional and advanced energy systems based on gas turbine and microturbine technology. This paper presents the theoretical background and the organization of the tool, showing the validation of results for a conventional recuperated microturbine.