The subject of this school, ‘The Origin and Evolution of Galaxies’, should include ‘Clusters of Galaxies’ in the title! Not only is galaxy clustering an essential aspect of the galaxy formation process, but I shall show that there is considerable evidence that galaxy morphology is not independent of is cluster environment. To understand galaxy evolution we must recognise the environmental effects, and before we can do that we have to recognise the galaxy cluster themselves. In these lectures I shall address this aspect of the problem, and I shall restrict the discussion to the observational data based on optical studies of rich clusters of galaxies. Moreover, I shall leave the theoretical explanations of the data to the imagination of the reader: relatively little theoretical work has been done on the environmental aspects of galaxy formation and I hope that this summary may provide some encouragement for theorists.