An event-by-event Monte Carlo simulation code for track structure studies is described. In the present form the code transports protons (âˆ¼0.3â€“10 MeV) and electrons (âˆ¼10 eVâ€“10 keV) in a water medium in the gas phase approximation. For the type of particles and energy range considered, ionization, electronic excitation and electron elastic scattering are the most important collision events accounted for in the transport simulation. Efforts were made to ensure that the analytic representation of the various interaction cross sections rests on well established experimental data and theory. For example, the secondary-electron spectrum as well as partial and total ionization cross sections are represented by a semitheoretical formulation combining Betheâ€™s asymptotic expansion and binary-encounter theory.