The ability to understand and effectively interact with others is a critical determinant of academic, social, and life success (DiPerna & Elliott, 2002). An area in particular need of scalable, feasible, usable, and scientifically sound assessment tools is social-emotional comprehension, which includes mental processes enlisted to encode, interpret and reason about social and emotional information. Social-emotional comprehension includes the abilities to infer others emotions from nonverbal cues, to take others' perspectives, to solve social problems, and to enlist cognitive strategies involved in self control. Well developed social-emotional comprehension is associated with academic, social, behavioral, and other important life outcomes (McKown, Allen, Russo-Ponsaran, & Johnson, 2013; McKown, Russo-Ponsaran, Johnson, Russo, & Allen, 2015; McKown, Russo-Ponsaran, Allen, Johnson, & Warren-Khot, 2015). However, there are few tools educators can use to evaluate their students' social-emotional comprehension and use findings to guide instruction. To address the need for direct assessments of social-emotional comprehension, the authors developed a web-based system called SELweb. SELweb assesses four dimensions of social emotional comprehension, three of which are adapted from Lipton and Nowicki (2009)'s model. "Social Awareness," the ability to understand others' emotions, draws on research on nonverbal communication (Nowicki & Duke, 1994). "Social Meaning," the ability to interpret others' mental states, draws on research on theory of mind and perspective-taking (Happé, 1994; Wellman & Liu, 2004). "Social Reasoning," the ability to reason about social problems, draws on social information-processing research (Bauminger, Edelsztein, & Morash, 2005; Crick & Dodge, 1994). Extending the Lipton and Nowicki (2009) model of social-emotional comprehension, the authors include "Self-Control," which includes mental processes involved in delaying gratification and controlling emotions to achieve a goal (Duckworth, 2011). This paper summarizes lessons learned from a four year IES-funded Goal 5 project to develop and evaluate SELweb. In keeping with the theme of this year's SREE conference, this paper will review scientific and practice lessons learned. In service to summarizing the science, the paper will summarize key findings from two studies of the psychometric properties of SELweb. In service to summarizing practice lessons learned, the paper will describe strategies used throughout the research and development process, and mid-course corrections made along the way, to ensure the practical usefulness of the research process and research data to education partners. Educators reported that SELweb was easy to use and informative, but that they did not always know how to use assessment findings to guide action. In addition, they provided important, and sometimes surprising, feedback about SELweb features they found particularly useful, and the features they would like to see incorporated into the system to maximize its usefulness. Tables and figures are appended. [SREE documents are structured abstracts of SREE conference symposium, panel, and paper or poster submissions.]