The concept of inference is not only the central notion of logic, it is also the key to our understanding of rationality and knowledge. A valid inference delivers a ground for the conclusion given that there are grounds for the premisses. When it is deductive, it even guarantees the truth of the conclusion given the truth of the premisses. As Aristotle already put it: the conclusion follows by necessity from the premisses. The nature of this modal notion of necessity is however a crucial issue. It has long been thought that it can be explained in terms of logical consequence as this concept was defined by Bolzano and Tarski. I shall argue that in this way we do not get an adequate definition of what it is for an inference to be valid and that, instead, we have to regard inferences as acts by which we transform grounds for premisses to grounds for the conclusion.