Divertor detachment is currently assumed to be a fundamental pre-requisite for the successful operation of future fusion reactors. Only by partially detaching the divertor in the regions of highest power flux density can high performance tokamak operation be made compatible with the technological limits set by the thermo-mechanical properties of surfaces in contact with the plasma. Although the various physics components of the detachment process are thought to be well known, their relative importance and the degree to which each may affect the others, thus determining the final detached state, cannot in general be deduced from any simple analytic approach. Instead, sophisticated two and three dimensional interpretative and predictive code packages have been developed within the fusion community to model the scrape-off layer (SOL) and divertor plasmas of magnetic confinement devices. One of these, the SOLPS code, has for some time been employed as a tool for the design of the divertor in the next step tokamak reactor, ITER. In reality, however, these codes have not, in general, been fully validated against experimental observations from current tokamaks, in particular with regard to divertor detachment. This thesis aims to contribute to such validation by thorough comparisons between numerical simulations, using the SOLPS5 (plasma fluid, Monte Carlo neutral) code pack- age and experimental characterization of the detachment process in two very different tokamaks, TCV and JET. The approach taken has been to test the code against experiment, not only for two machines with a vast difference in size and divertor geometry, but also for plasma operation with either deuterium or helium fuel. Changing the fuel species in a tokamak containing significant graphite first wall components as do TCV and JET, dramatically modifies the impurity production mechanism but also the important atomic physics processes at work, both of which influence the detachment threshold. In TCV, divertor detachment in the simplest of situations is experimentally observed to be anomalous and could not be explained by the first attempts at code modeling prior to this thesis. Evidence is presented for the detachment anomaly being directly linked to enhanced interaction between the graphite main chamber walls at high plasma density due to anomalous convective radial transport. Such interaction is not well modeled by the code and the results presented in this thesis highlight an important area in which the complexities of the real situation are inadequately represented in the numerical model. This work also constitutes the first known application of the SOLPS code to tokamak simulation with consistent modeling of molecular hydrocarbons. Indeed, they are found to be important in producing high degrees of numerical detachment. In JET, experimental data from high density helium plasma operation have been successfully modeled, constituting the first ever simulations of pure He discharges on this machine. Helium detachment is very different to that in deuterium, due in large part to the absence of carbon chemistry. The simulation results demonstrate this together with strong evidence for conclusions to be drawn concerning the principal mechanisms driving the detachment. Similar good agreement is obtained between code and experiment for helium operation on TCV and a comparison of code results between the two devices demonstrates how divertor geometry can have a significant impact on the detachment behavior.