Although young children frequently have more difficulty in correctly identifying main ideas than do older children and adults, the reasons underlying this difference in performance have usually not been investigated. The present study was designed to formulate a task involving both the form and content of good main ideas, so that the development of skills used in their identification could be examined in greater detail. Four categories of main idea statements were developed from a joint theory combining research on story grammar kernels with research on summarization rules. These categories were then applied to a series of simple narratives from the WISC-R picture arrangement task. College students (Experiment 1) and 2nd, 5th, and 8th grade students (Experiment 2) examined the narratives and rank ordered the four alternatives for each story. It was found that adults confirmed the predicted ordering of the alternatives and that, with age, children improved in their identification of the best main idea statement, and developed in their ability to distinguish both between important and unimportant story elements and between superordinate and subordinate types of statements.