Over the last several decades, researchers have studied many and varied aspects of rater cognition. Those interested in pursuing basic research have focused on gaining an understanding of raters' thought processes as they score different types of performances and products, striving to understand how raters' mental representations and the cognitive strategies and rating styles they employ influence their judgments. Researchers have also carried out studies to identify the prominent features of those performances or products to which raters attend and then determine how raters differentially weigh those features when assigning ratings. One of the major contributions of this line of work has been the construction of various theoretical models that hypothesize sequences of mental activities in which raters engage to reach their judgments. Other researchers with a somewhat more applied bent have focused on isolating factors that are associated with raters assigning accurate, reliable, and fair ratings and on gaining insights into how various factors impede rater functioning. To that end, researchers have studied rater background variables (e.g., years of teaching experience, prior experience as a rater), as well as aspects of assessment contexts (e.g., performance task and rubric design, centralized and online scoring models) and their effects on raters' ratings. A few researchers have studied the effectiveness of various approaches to rater training, seeking to identify aspects of rater cognition that appear malleable to training and to determine how training might improve raters' judgmental processes. Additionally, some researchers have used statistical models to analyze sets of rating data in an attempt to detect and measure various rater effects that may signal problematic rater cognitive functioning and to determine whether such effects for individual raters are constant across time or transient. This article discusses some of the challenges rater cognition researchers face as they plan new research efforts.