  We have developed a new semi-analytic model for the formation and evolution
of structure on galaxy, group and cluster scales. The model combines merger
trees with a detailed, spatially resolved description of the dynamical
evolution of halo substructure. It reproduces the results of numerical
simulations of mergers and halo formation at a fraction of the computational
expense. We use this model to study the formation of the dwarf galaxies in the
Local Group and the structural components of the Milky Way. We find that
reionization can explain the scarcity of Local Group dwarfs, although the
reionization epoch is constrained to be high -- z_ri >= 12. We also find that
disk disruption at recent times is rare, such that many galactic disks have an
old, thin component.
