The ascent to the One lies at the heart of Plotinus' philosophy and of the Neoplatonism which started with him. The realisation that such an ascent already plays a central role in Plato is one of the most important insights of recent work on Platonic thought. This book draws the consequences of that discovery, and redefines the relationship between Plato and Neoplatonism. It combines the first comprehensive account of Plotinus' theory of the Absolute with a reconstruction of Plato's henology. In doing so it reveals the close relationship between the two thinkers and also demonstrates the enduring philosophical relevance of the topic.