Are there things we shouldn't know? It has been argued that decisions regarding the topics of scientific research and the publication of research results are purely methodological and that any moral considerations relate only to research methods and uses of the acquired knowledge. The arguments advanced in favor of this view appeal to the moral neutrality of scientific knowledge and the intrinsic value of truth. I argue that neither is valid. Furthermore, I show three cases in which a scientist's decision to begin an investigation clearly has moral relevance: (1) when initiating an investigation would create circumstances that threaten some noncognitive values; (2) when achieving certain knowledge would jeopardize the existence of the individual's private sphere; and (3) when there are reasons to think that humanity is unwilling to accumulate knowledge. These cases do not prove the existence of some intrinsically "morally forbidden topics", but show that the moral permissibility of a given investigation is not guaranteed a priori but must be judged in the same way as its methodological soundness is judged. Judgments regarding research topics have both methodological and moral aspects and these two cannot be separated under the risk of distorting science. Making such judgments requires knowledge not only of scientific methodology, but also of its social and philosophical implications. Philosophy is necessary to do good science. My search for an answer to the title question is limited to science, which is the main source of our knowledge of the world, and its moral dimension. To know something in a scientific way it is necessary to investigate relevant topics with scientific means. There are then arguments that... at the center of the paper... DODahlstrom. Nature and scientific method. Washington, DC: The Catholic University of America Press 1991. 95-105. Gaerdenfors P. 'Is there anything we shouldn't want to know?' in: JEFensted (ed.), Logic, Methodology, and the Philosophy of Science, New York: Elsevier 1990. 63-78. Glass, B. 'The ethical foundations of science' in: Bulger, R. E. et al. (ed.). The ethical dimension of biological sciences. Cambridge University Press 1993. 43-55.Herrnstein RJ and Wilson J. Q, Crime and Human Nature, New York: Simon and Schuster 1985.Rescher, N. 'Forbidden Knowledge' in: Forbidden Knowledge and Other Essays on the Philosophy of Cognition , Dordrecht: Reidel 1987. 1-16.Verhoog, H. Genetic modification of animals. Should science and ethics be integrated? in: A. Lekka-Kowalik and D. Schulthess (eds.). Forbidden knowledge. The Monist 79 (2) 1996.