This article analyzes details of the cognition involved when people use spreadsheet software, a task that is both a major microcomputer application and a cognitively intense task. This task is analyzed in terms of the GOMS model (Card, Moran, & Newell, 1983), to test the generality of the model and to extend its set of parameters. We found that people using two seemingly similar spreadsheet applications, Lotus 1-2-3 and Multiplan, require very different amounts of time to accomplish the same tasks. Experienced users of Lotus 1-2-3 took far longer to complete the same four tasks than experienced Multiplan users did. It was found that some of additional time was due to the fact that Lotus 1-2-3 offers users a choice of two general methods to enter formulas. Lotus requires that the user decide which to use; this decision takes time. And, when the users type the address of the cell in which values reside instead of using the cursor to point to it, they pause a long time before typing each entry. Presumably they are scanning the screen and calculating the coordinates to type in during the pause. Again, these cognitive processes take time. In an analysis of a second task-adjusting the column width-there was substantial evidence that the performance changes when a method is repeated in close succession. This repetition affects the parameters that reflect the time it takes to retrieve command parts from memory. When the parameters for scanning, decision, and repetition were added to the keystroke analysis of our task, we found remarkable correspondence with the basic parameters from the Card et al. (1983) original work: The keystroke times and mental preparation times from their original experiments were very close to the estimates of those same parameters in our tasks. However, in our analysis of the spreadsheet task, we expanded the parameter set in the keystroke model to account for performance in tasks that require substantial planning, scanning, and repetition.