Readers often struggle to identify the main ideas in expository texts. Existing research and instruction provide some guidance on how to encourage readers to identify main ideas. However, there is substantial variability in how main ideas are operationalized and how readers are prompted to identify main ideas. This variability hinders identification of best practices for instruction and intervention. The goal of the current series of experiments was to systematically examine the extent to which different tasks (e.g., selecting main ideas vs. deleting details) and different operationalizations of main ideas (e.g., "important ideas" vs. "main ideas") influenced adult readers' identification of sentences containing main ideas as they read 11 expository texts. Across experiments, the results showed that readers were generally unreliable in identifying main idea sentences; however, they were more reliable when they were instructed to select main idea sentences compared to when they were instructed to delete sentences comprised of details, and more skilled readers were more reliable than less skilled readers. The findings from the current experiments challenge existing instructional approaches and call for additional research to better understand readers' main idea selection.