What does it mean for a mentoring program to succeed? Most evaluations focus on participants' perceptions of success. Few studies employ an independent measure of the intended outcomes of the program, and fewer still examine both participant satisfaction and achievement. This article presents an example of how comparing data on perceived and achieved success can foster new insights into the trade-offs inherent in program design and improvement. Based on an analysis of survey and interview data from an online mentoring program in which 17 history experts supported 108 high school students in historical research, it was found that mentees' satisfaction did not correlate with their achievement of intended program outcomes. Further analysis illuminated design trade-offs between the aspects of the online mentoring program that supported perceived success and those that supported intended outcomes. The authors argue that examining such trade-offs in other mentoring programs may facilitate their improvement over time. (Contains 4 tables and 1 figure.)