Comprehending instructions is a frequent task in everyday life. A set of instructions consists of a series of steps that indicate how to produce a final state. In cognitive terms, following a set of instructions implies building an internal representation of the procedure depicted in the message, with the aim of reaching the final state accurately and fast. There fore, comprehending instructions is intrinsically associated with executing them efficiently. Whereas previous research on the area has focused mainly on traditional indicators of comprehension, such as reading times and recall, this study incorporates measures of the actual execution (i.e., the time spent to perform the instruction and the error rate) in addition to more traditional processing measures. Additionally, this study analyzes the potential effects of both cognitive and design features on comprehension. A core aspect in the study of processing instruc tions is to determine which factors are more effec tive in helping users to achieve consistent and reproducible results. This experiment examined the effect of the presentation format of the instructions, and individual factors such as expertise and verbal and visuo-spatial work ing memory capacity on comprehending and executing a set of instructions. Dependent variables were the mean time to study the instruc tions, the mean time to assembly the objects, and the percentage of errors during assembly. Seventy-two advanced (fourth-year) college students, half in scientific-technical fields, and the other half in humanities, participated voluntarily. All participants completed an experimental assembly task which required building Lego objects. Each one consisted of nine experimental trials of five-steps instructions in one of the three presentation formats (pictorial only, verbal only, or combined verbal and pictorial information assigned randomly), followed by verbal (Letter Number Sequencing, WAIS-III and Reading Span), and visuo-spatial (Spatial Span, WMS-R and Mazes, AWMA) working memory tests. Each individual session lasted about an hour. Three ANCOVAS were conducted to examine the effect of presentation format and expertise on study time, execution time and error rates (%). Verbal and visuo-spatial working memory capacity measures were included as co-variables. The presentation format had effects both on the time to study and the time to perform the instructions: verbal only presentations were processed slower than presentations including pictures (image only or multimedia), and were slower than multimedia in assembly. Expertise, on the other hand, had its effect on execution errors, favoring students of scientific and technical background, partialing out individual differences such as working memory capacity. There was a tendency for students with higher verbal working memory to process instruc tions faster, and for those with higher visuo-spatial working memory to assembly objects more precis ely. Overall, these results suggest that images are more effective in representing procedural instructions, since the pictorial and multimedia formats seemed to facilitate the comprehension of the instru ction. Results also suggest that the combined format of sentences and pictures enhanced efficiency during assembly, as evidenced by the faster execution time observed in the multimedia condition. This pattern of results is in line with the multi media advantage and the efficacy of images found with other text types, while extending it to procedural instructions. Additionally, visuo-spatial working memory and expertise seemed to influence the execution of the instructions. Verbal working memory capacity also seemed to be in volved when studying and processing the instruct ions, regardless of the presentation format, but not when executing the procedure. In conclusion, in order to maximize their efficiency, the format in which the instructions will be presented as well as the cognitive characteristics (i.e., working memory capacity and expertise level) of the potential users should be incorporated into the design of procedural instructions. [ABSTRACT FROM AUTHOR]