This multiple-case study examined how advanced learners solved a complex problem, focusing on how their frequency and application of cognitive processes contributed to differences in performance outcomes, and developing a mental model of a problem. Fifteen graduate students with backgrounds related to the problem context participated in the study. Data sources included direct observation of solution operations, participants' think aloud and stimulated recalls as they solved the problem, as well as solution scores indicating how well each participant solved the problem. A grounded theory approach was used to analyze stimulated recall and think aloud data. A set of thirteen cognitive processes emerged in the coding and were tallied for each participant. Individual cases were then grouped into clusters that shared similar frequencies of prior knowledge activation, performance outcomes, and tool use behaviors. Each cluster was profiled from least to most successful with descriptive accounts of each cluster's approach to solving the problem. A cross cluster analysis indicated how learners' cognitive processes corresponded with problem solving operations that revealed thresholds of knowledge development and formed an integrated mental model of the problem. The findings suggested that mastering problem solving operations within each threshold enhanced the learners' conceptual awareness of where to apply cognitive processes and increased the combinations of cognitive processes they activated at higher thresholds of knowledge development. The findings have implications for anticipating where novices need support within each threshold of knowledge development during complex problem solving.