Catheterization as a medical technique has been in practice for more than two millennia. While it first began as an experimentation with animals, it gradually became part of human treatments. The procedure of cardiac catheterization in humans was first put to clinical use more than five decades ago and has undergone many changes since then. What was once a purely experimental technique is now one of the most common invasive medical procedures in Europe and North America, with more than eighty-percent of those procedures performed to diagnose suspected heart disease. Animal cardiac catheterization was first accomplished in 1844 with a horse. The characterization of the human heart was first accomplished by a German medical student in 1929. By the 1940’s, the practice had become improved and was starting to become common practice in a few hospitals in North America and Europe. By entering a catheter through an arm vein, surgeons were able to reach the atrium of the heart. Today, cardiac catheterization is accomplished with a great deal of skill and technology, but is much easier and less dangerous than when it first was being used. It is the purpose of this paper to examine the history of cardiac catheterization and its import to medicine today.