The Internet is increasingly being used as a medium for educational software in the form of miniature applications (e.g., applets) to explore concepts in a domain. One such effort in mathematics education, the Educational Software Components of Tomorrow (ESCOT) project, created 42 miniature applications each consisting of a context, a set of questions, and one or more interactive applets to help students explore a mathematical concept. They were designed by experts in interface design, educational technology, and classroom teaching. However, some applications were more successful for fostering student problem-solving than others. This article describes the method used to mine a subset (25) of these applets for design principles that describe successful learner-centered design by drawing on such data as videos of students using the software and summaries of written student work. Twenty-one design principles were identified, falling into the categories of motivation, presentation, and support for problem solving. The main purpose of this article is to operationalize a method for post hoc extraction of design principles from an existing library of educational software, although readers may also find the design principles themselves to be useful.