An evidence-based connection between adult educator professional development (PD) and learner outcomes is difficult to document, yet there is an intuitive assumption that professional development is linked to better teaching and learning outcomes. The field appears to be shifting away from one-shot PD to practitioner engagement in sustained, collaborative knowledge construction. PD workshops have many limitations, such as location, time, and costs. Single workshops may be a useful way to provide information and raise awareness of issues, but changes in behavior and practice require longer-term approaches. Collaborative practitioner inquiry/research is being used extensively in PD in adult basic education, and evaluations have demonstrated significant changes in practice. However, such approaches are difficult to implement on a large scale, and their effects are subject to numerous other influences. Rather than asking which PD method or model is more effective, professional development should be considered a transformative process of critical reflection that leads to thinking and acting differently. How to demonstrate the impact of professional development on learning outcomes is in dispute. Using learner achievement as a measure is problematic, because factors associated with the practitioners, learners, programs, and PD system also influence outcomes. A more inclusive definition of PD and a broader conception of impact can help in assessing the effectiveness of professional development. (Contains 16 references.) (SK)