The next generation will be better prepared to cope with the daunting sustainability challenges if education for sustainable development is being taught and learned across educational sectors. K-12 school education will play a pivotal role in this process, most prominently, the teachers serving at these schools. While pre-service teachers' education will contribute to this transition, success will depend on effective professional development in sustainability education to teachers "currently" in service. Arizona State University has pioneered the development and delivery of such a programme. We present the design principles, the programme, and insights from its initial applications that involved 246 K-12 in-service teachers from across the USA. The evaluation results indicate that due to participation in the programme, sustainability knowledge, perception of self-efficacy, inclusion of sustainability in the classroom, modelling of sustainable behaviours, and linking action to content all increased. We conclude with recommendations for the widespread adopting of the programme.