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This dissertation presents numerical simulation results of the interaction of Mars ionosphere with the solar wind plasma using our new multi-fluid model, and the application of the model to Venus. Our study starts with a multispecies Magnetohydrodynamics (MHD) single- fluid model. It contains four ion species, H+, O+2 , O+ and CO+2 and includes an extensive chemistry model. We modify our existing single fluid model and solve the multi-fluid equations. This results in the ion species being decoupled, that is each one has its own density, velocity and pressure. We use a spherical adaptive grid system to obtain a very good altitude resolution (10km) and set the lower boundary at 100 km altitude in order to include the ionospheric region in our simulation. Our results show clear asymmetries in the X-Z plane due to the effect of the electric field on the decoupled ions. These asymmetries, similar to the ones observed by kinetic models, could not be observed by the single fluid MHD model and show us a new distribution of the ions around the planet, a distinct magnetic field configuration and different escape fluxes. The model results agree well with Mars Global Surveyor (MGS) and Viking observations. We use our Mars model to study the effect of different upstream conditions, the hot ionospheric oxygen corona and the crustal magnetic field. These applications show a clear change in the bow shock location and the ion distribution. Finally, we successfully apply our multi-fluid model to Venus thus illustrating the versatility of our model. We reproduce the physical processes (asymmetries) calculated in Mars' case and observe, as expected, that kinetic effects are less pronounced in Venus' case. We use our model to study the effects of different solar wind conditions and we compare our results to observations by Pioneer Venus Orbiter (PVO) and Venus Express (VEX). Our results reproduce observed bow shock positions and show a reasonable fit to the data.