An investor who wants to invest a certain amount and to whom a lot of more or less risky alternatives arise would divide this amount among several securities. He makes this portfolio decision because of his expectations with regard to these assets which result from the information available to him. If the investor obtains additional information, then his knowledge would improve and, therefore, the portfolio decision made by him. Accordingly, he will be ready to accept certain costs related to the information procurement. The value of information indicates the maximum tolerable information costs, and its knowledge, therefore, enables - by comparing with the actual information costs - to evaluate the profitability of an information procurement. In this book, the value of information for the problem of portfolio planning is explicitly determined, namely as well for the case of fixed prices not influenced by the information activity as within the scope of a market model. These explicit results allow several conclusions, in particular about the influence of preknowledge, risk aversion, information precision and information dissemination on their value. The Bayesian decision theory is the basis for this paper. CorresΒ pondingly, a subjective concept of probability is underlying, and the information processing and evaluation is understood in a staΒ tistical sense. As one might expect, the question about the correctΒ ness of an information is not treated, although manipulating the asset prices by deliberate dis information can be observed in pracΒ tice and is, certainly, an interesting problem.