The halo of the Milky Way provides unique elemental abundance and kinematic information on the first objects to form in the Universe, and this information can be used to tightly constrain models of galaxy formation and evolution. Although the halo was once considered a single component, evidence for its dichotomy has slowly emerged in recent years from inspection of small samples of halo objects. Here we show that the halo is indeed clearly divisible into two broadly overlapping structural components-an inner and an outer halo-that exhibit different spatial density profiles, stellar orbits and stellar metallicities (abundances of elements heavier than helium). The inner halo has a modest net prograde rotation, whereas the outer halo exhibits a net retrograde rotation and a peak metallicity one-third that of the inner halo. These properties indicate that the individual halo components probably formed in fundamentally different ways, through successive dissipational (inner) and dissipationless (outer) mergers and tidal disruption of proto-Galactic clumps.Astronomers have long sought to constrain models for the formation and evolution of the Milky Way (our Galaxy) on the basis of observations of the stellar and globular cluster populations that it contains. These populations are traditionally defined as samples of objects that exhibit common spatial distributions, kinematics and metallicities (the age of a population, when available, is also sometimes used). Metallicity is taken by astronomers to represent the abundances of elements heavier than helium, which are only created by nucleosynthesis in stars-either internally via nuclear burning in their cores or externally during explosive nucleosynthesis at the end of their lives. The earliest generations of stars have the lowest metallicities, because the gas from which they formed had not been enriched in heavy elements created by previous stars and distributed throughout the primordial interstellar medium by stellar winds and supernovae.Previous work has provided evidence that the halo of the Milky Way may not comprise a single population, primarily from analysis of the spatial profiles (or inferred spatial profiles) of halo objects [1][2][3][4] . A recent example of such an analysis is the observation of two different spatial density profiles for distinct classes of RR Lyrae variable stars in the halo 5 . In addition, tentative claims for a net retrograde motion of halo objects by previous authors supports the existence of a likely dual-component halo [6][7][8][9][10] . The central difficulty in establishing with confidence whether or not a dichotomy of the halo populations exists is that the past samples of tracer objects have been quite small, and usually suitable only for consideration of a limited number of the expected signatures of its presence.In the present work, we examine this question in detail using a large, homogeneously selected and analysed sample of over 20,000 stars, originally obtained as calibration data during the course of the Sloan Digital Sk...