Holography is a fruitful concept in modern physics. However, there is no generally accepted definition of the term, and its significance, especially as a guiding principle in quantum gravity, is rather uncertain. The present paper critically evaluates variants of the holographic principle from two perspectives: their relevance in contemporary approaches to quantum gravity and in closely related areas; their historical forerunners in the early twentieth century and the role played by past and present concepts of holography in attempts to unify (...) physics. By combining these two perspectives a certain depth of focus is gained which allows us to draw some tentative conclusions about what might be reasonable aspirations and prospects for holography in quantum gravity. By the same token, we will have a brief and critical look at wider philosophical interpretations of the term. (shrink)