  We present our measurements of the deuterium to hydrogen ratio (D/H) in QSO
absorption systems, which give D/H = 3.40 $\pm 0.25 \times 10^{-5}$ based on
analysis of four independent systems. We discuss the properties of two systems
which provide the strongest constraints on D/H. We outline the systematic
effects involved in measurements of D/H and introduce a sophisticated method of
analysis which properly accounts for these effects.
