This paper presents a cognitive account of both the advantages and disadvantages of direct manipulation interfaces, i.e., the use of icons to manipulate and interact directly with data rather than writing programs or calling on a set of statistical subroutines. Two underlying phenomena that give rise to the sensation of directness are identified. One is concerned with the information processing distance between the user's intentions and the facilities provided by the machine; the second is concerned with the relation between the input and output vocabularies of the interface language. It is noted that direct manipulation requires that the system provide representations of the objects of interest that behave as if they were the objects themselves in order to create the sensation of directness of manipulation. Concluding remarks admit the complexity of understanding direct manipulation interfaces, but stress that a properly designed system should be easy for the user to learn, enabling people to spend their time learning the task domain rather than the computer system. Lists of 20 references and 30 reports by people in the Institute for Cognitive Science complete the document. (JB)