  The Solar System formed about 4.6 billion years ago from a condensation of
matter inside a molecular cloud. Trying to reconstruct what happened is the
goal of this chapter. For that, we put together our understanding of Galactic
objects that will eventually form new suns and planetary systems, with our
knowledge on comets, meteorites and small bodies of the Solar System today. Our
specific tool is the molecular deuteration, namely the amount of deuterium with
respect to hydrogen in molecules. This is the Ariadne's thread that helps us to
find the way out from a labyrinth of possible histories of our Solar System.
The chapter reviews the observations and theories of the deuterium
fractionation in pre-stellar cores, protostars, protoplanetary disks, comets,
interplanetary dust particles and meteorites and links them together trying to
build up a coherent picture of the history of the Solar System formation. We
emphasise the interdisciplinary nature of the chapter, which gathers together
researchers from different communities with the common goal of understanding
the Solar System history.
