Coronary stenosis is one of the most common heart diseases. Though not much is known concerning the growth of the stenosis in the coronary arteries, it is nevertheless possible to study the effects of the stenosis on the blood flow, and thus to determine its influence on myocardial perfusion. In summary, this thesis is concerned with the effects of the stenosis on left coronary blood flow. To determine these effects it has been necessary in a first step to understand left coronary blood flow and its regulation. This is done by computing coronary blood flow as a function of the hydraulic power delivered by the heart. In a second step the hydrodynamic effects of the stenosis have been computed and experimentally tested in vitro. It has been found that the resistance to the flow presented by the stenosis is a strong function of the percentage of stenosis, its inner radius and its length. Apparently. the geometrical inlet shape of the stenosis is not an important factor. It has been shown also that for given conditions a 75% stenosis can present a high enough resistance to impede myocardial blood flow such as to lead to myocardial infarction. This is true even if the required blood supply is far from being excessive.