The Aquarius Project is a comprehensive study of the subhalos within galactic halos, using the largest particle simulations of a Milky Way-sized dark matter halo. The project includes simulations of six ultra-highly resolved Milky-Way sized halos to estimate the halo-to-halo scatter in substructure statistics. The simulations resolve nearly 300,000 gravitationally bound subhalos within the virialized region of the halo, with mass resolutions up to 1800 times higher than previous studies. The results show that subhalos contain up to four generations of substructures, but the overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum, the mass fraction in substructure is predicted to be well below 3% within 100 kpc and below 0.1% within the Solar Circle. The inner density profiles of subhalos are well fit by gently curving Einasto profiles, and the mean concentrations of isolated halos are accurately described by the Neto et al. fitting formula. However, subhalos are more concentrated than field halos, with a characteristic density typically 2.6 times larger and increasing with decreasing distance from the halo center. The project addresses key questions in cosmology, such as the nature of dark matter and the distribution of substructures within galactic halos, and provides insights into the detectability of dark matter annihilation and the structure of central density cusps.The Aquarius Project is a comprehensive study of the subhalos within galactic halos, using the largest particle simulations of a Milky Way-sized dark matter halo. The project includes simulations of six ultra-highly resolved Milky-Way sized halos to estimate the halo-to-halo scatter in substructure statistics. The simulations resolve nearly 300,000 gravitationally bound subhalos within the virialized region of the halo, with mass resolutions up to 1800 times higher than previous studies. The results show that subhalos contain up to four generations of substructures, but the overall substructure mass fraction is much lower in subhalos than in the main halo. Extrapolating the main halo's subhalo mass spectrum, the mass fraction in substructure is predicted to be well below 3% within 100 kpc and below 0.1% within the Solar Circle. The inner density profiles of subhalos are well fit by gently curving Einasto profiles, and the mean concentrations of isolated halos are accurately described by the Neto et al. fitting formula. However, subhalos are more concentrated than field halos, with a characteristic density typically 2.6 times larger and increasing with decreasing distance from the halo center. The project addresses key questions in cosmology, such as the nature of dark matter and the distribution of substructures within galactic halos, and provides insights into the detectability of dark matter annihilation and the structure of central density cusps.