In this article, we discuss case decision predictors, algorithms which, given some features of a legal case predict the outcome of the case (i.e. the decision of the judge). We discuss whether, and if so how, such prediction algorithms can be used to support judges in their decision making process. We conclude that case decision predictors can only be useful in individual cases if they can give legal justifications for their predictions, and that only these legal justifications are what should matter for a judge