Introduction It is hard to think of any scientific endeavour that has changed more during the past century than cosmology. A hundred years ago other galaxies were thought of as gas clouds probably somewhere in our galaxy. The Universe was regarded as static and never-changing, with no beginning and no end. The birth of the Universe from a quantum sized object was never considered. The expansion of the Universe and the description of the effects of gravity as the curvature of space-time were still to come. The synthesis of elements in the stars was not understood and the Microwave Background Radiation was still to be discovered. It is not only cosmology that has been changed drastically by these discoveries but our view of the world and man's place in it has been altered beyond all recognition. Cosmology as a science has come of age and is no longer considered to be part of religion or philosophy. Let us look at what these discoveries were and why they changed cosmology so dramatically.