  We describe the birational geometry of the moduli space S_g^{-} of odd spin
curves (theta-characteristics) for all genera g. The odd spin moduli space is a
uniruled variety for g<12, and of general type for g at least 12. Furthermore,
for g<9 we use the existence of Mukai models of the moduli space of curves, to
prove that S_g^{-} is unirational. Our results show that in genus 8, the odd
spin moduli space in unirational, whereas its even counterpart is of Calabi-Yau
type.
