We have known for almost three decades that the steps in a computer that require a minimal energy expenditure, that cannot be avoided by any means, are those that discard information. For more than half that period we have known that such steps are not essential; computation can be carried out through a sequence of logical 1:1 mappings. Computation, therefore, can be carried out with arbitrarily little dissipation per step, if done sufficiently slowly. Much more recently it has been emphasized that measurement and communication are similar to computation; it is only the information-discarding steps that have a lower bound on the dissipation. Such steps are not required in communication. In measurement, as shown by Bennett, they only become essential when we reset the meter for its next (or first) use. This paper is not a detailed exposition of all this, but only an annotated guide to the existing literature.