The common iliac vein is a large paired vessel in the human body formed by the fusion of the external iliac and internal iliac veins.

It begins anterior to the sacroiliac joint, where the two veins mentioned above join. The common iliac vein passes obliquely upward to the fifth lumbar vertebra (L5), a little to its right. At this point both common iliac veins (right and left) come together and form the inferior vena cava. 

On its way, the common iliac vein collects the iliolumbar and sometimes the lateral sacral veins, as well as an unpaired median sacral vein, which typically flows into the left common iliac vein.

Overall, the common iliac veins collect venous blood from the pelvis and the lower limbs.