This paper focuses on description and evaluation of a Web-based professional development system for teachers centered on the collection and use of frequent student performance information in algebra for instructional decision making. With U.S. federal grant funding, researchers developed Web-based instructional modules, which provided interactive instruction in the administration and scoring of three types of algebra measures. Modules also addressed use of the data management system for storing, displaying, and helping teachers to interpret data. Evaluation of this online system included feedback from nine special and general education mathematics teachers regarding their scoring accuracy, satisfaction with the content and usability of the system, and change in their knowledge about progress monitoring. In addition, a hypothetical student case study illustrates the use of this online system for informing instructional decisions within the context of a school's model for response to intervention. In this framework, teachers use progress monitoring data to identify students who are struggling, so increasingly more intensive instruction may be delivered to better meet student needs. This illustration depicts how an algebra teacher may use the online system to identify students who need more instructional support and to monitor students' progress.