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 .