This quasi-experimental study examined the effectiveness of Assessing One and All (AOA), a Web-based teacher professional development program. AOA was designed to enhance teachers' assessment literacy and skills in general and inclusive educational assessment practices. Fifty-five teachers from Arizona, South Carolina, and Wisconsin participated in the study. Repeated-measure analysis, matched-pair t-tests, and qualitative analysis were used to examine whether (a) AOA effectively enhanced teachers' knowledge and self-efficacy in inclusive assessment practices, and (b) teachers accepted the online course as an alternative to traditional professional development programs. Despite some limitations in the design and execution of the study, AOA improved teachers' knowledge and self-efficacy of assessment issues and can serve as an efficient and effective professional development tool. (Contains 1 figure and 6 tables.)