Teachers are not typically involved as participatory designers in the design of technology-enhanced learning environments. As they have unique and valuable perspectives on the role of technology in education, it is of utmost importance to engage them in a participatory design process. Adopting a case study methodology, we aim to reveal in what ways teachers work as participatory designers and define conditions that support teachers in that. Two initiatives of participatory design in Canada and Singapore were investigated. Design materials, transcripts of design meetings, and interviews with teachers were qualitatively analyzed. Case study 1 (Canada) showed that two teachers participating in software design for an astronomy curriculum contributed by suggesting new design features, introducing pedagogical requirements, and providing feedback on prototypes or design ideas. It appeared essential that teachers feel that their ideas were valued and respected in the entire process. In case study 2 (Singapore), six teachers contributed to the design of a mobile learning trail through: Theorizing and bridging knowledge building principles, collaborative prototyping, contextual inquiry of activity relevance and activity execution, and collaborative evaluation of technology integration. Teachers valued case study discussions with similar cultural contexts and visiting the learning site to design with contextual knowledge. From our case studies, it can be concluded that teachers contribute to the design processes by engaging in theoretical discussion, active participation in a design partnership, reflection about pedagogy and practice, and experimenting with enactment. Conditions that support teachers include support in emergent processes and an atmosphere of trust and inclusion.