The concept of a community of practice (CoP) is prevalent in several venues for teachers' professional development, especially in online environments. However, there are few descriptive accounts that effectively represent a CoP in a manner that will be of use to other designers. In order to illuminate potential difficulties which may arise when attempting to design a framework to characterize or to build a CoP, this study describes the dynamics of five dualities (specific areas of tension) that were identified during the design and testing period of the Inquiry Learning Forum (ILF), a Web-based community for teachers' professional development. During the three-year design trajectory of the ILF, these five dualities emerged from and characterized the interactions between the participating teachers and the site designers. As part of the data collection for this study, we conducted document analyses, interviews with designers, researchers, and teachers, and observations of online and face-to-face meetings. The findings of this study are intended to help future Web-designers both to better realize the full potential of online professional development environments and to avoid potential design development issues which may hamper the utility or participation rates in newly created CoPs. (Contains 2 figures and 2 tables.)