Streams, substructures and the early history of the Milky Way
  The advent of Gaia's 2nd data release in combination with large spectroscopic
surveys are revolutionizing our understanding of the Galaxy. Thanks to these
and the knowledge accumulated thus far, a more mature picture of the evolution
of the early Milky Way is emerging:
  * Two of the traditional Galactic components, i.e. the stellar halo and the
thick disk, appear to be intimately linked: stars with halo-like kinematics
originate in similar proportions, from a "heated" (thick) disk and from debris
from a system named Gaia-Enceladus. Gaia-Enceladus was the last big merger
event experienced by the Milky Way and probably completed around 10 Gyr ago.
The puffed-up stars now present in the halo as a consequence of the merger have
thus exposed the existence of a disk component at z ~ 1.8.
  * The Helmi streams, Sequoia, and Thamnos are amongst the newly uncovered or
better characterized merger events. Knowledge of their progenitor's properties,
star formation and chemical histories is still incomplete.
  * Debris' from different objects often overlap in phase-space. A task for the
next years will be to use spectroscopic surveys for chemical labelling and to
disentangle events from one another using dimensions other than only
phase-space, metallicity or [alpha/Fe].
  * These surveys will also provide line-of-sight velocities missing for faint
stars and more accurate distance determinations for distant objects. The
resulting samples of stars will cover a much wider volume of the Galaxy
allowing, for example, linking kinematic substructures in the inner halo to
spatial overdensities in the outer halo.
  * All the results obtained so far are in-line with expectations of current
cosmological models. Yet, tailored hydrodynamical simulations as well as
"constrained" cosmological simulations are needed to push our knowledge of the
assembly of the Milky Way back to the earliest times. [abridged]
