This thesis examined the ocular and systemic vascular variables in low tension glaucoma (LTG), primary open angle glaucoma (POAG) and normal control subjects to look at specific associations between vascular abnormalities and low tension glaucoma. The investigate tests carried out were as follows: 1. Pulsatile Ocular Blood Flow (POBF) was measured with an adapted pneumotonometer in the supine, sitting and erect positions. POBF was significantly lower in LTG, especially in the supine position, compared to normal. In POAG the POBF was significantly higher than normal and LTG. 2. Colour Doppler Imaging (CDI): There was an increased resistive index in the central retinal artery in both LTG and POAG. There was an increased resistive index in the ophthalmic artery of POAG patients compared to LTG and normals. There was a significant increase in peak systolic velocity (PSV) in the ophthalmic artery in the POAG group. 3. Peripheral Laser Doppler Velocimetry: Finger blood flow was quantified by laser Doppler velocimetry. The results showed a reduced flow in LTG patients following cold immersion, with a prolonged recovery time to baseline flow. 4. Rheology: The results showed significantly higher levels of prothrombin fragments F1+2 and D-dimer in POAG compared to LTG and normals. The fibrinogen level was higher in glaucoma than in normals. This indicates that the coagulation cascade and fibrinolysis pathway were activated in POAG. 5. Using discriminant analysis, two vascular variables were identified which could separate POAG, LTG and normal controls from each other with a 76% discriminate predictive power. These two variables were prothrombin fragments 1+2 (a measure of coagulation) and a prolonged recovery time to baseline finger blood flow following cold immersion (an index of vasospasm). The results show a reduced pulsatile ocular blood flow, an increase in vascular resistance in the central retinal artery and a tendency to digital vasospasm in LTG.