Online modes of teacher professional development (PD) have gained prominence in recent years for their potential to transform and expand access to high-quality resources and experiences that positively impact teachers' knowledge, beliefs, instructional practices, and ultimately, student learning. However, with the increasing demand for and availability of online offerings, there is still much to be learned about the effectiveness of these programs and the factors that contribute to their success. The National Science Foundation's (NSF) Discovery Research PreK-12 (DRK-12) program, which "seeks to significantly enhance the learning and teaching of science, technology, engineering, mathematics and computer science (STEM) by preK-12 students and teachers, through research and development of STEM education innovations and approaches," is poised to contribute in this area (NSF, 2017, p. 1). In fall 2016, Community for Advancing Discovery Research in Education (CADRE) (the resource network for NSF's DRK-12 program) identified 26 active projects in the DRK-12 portfolio that were researching and/or developing online or blended teacher PD programs. Between February and September 2017, awardees representing 11 projects participated in a series of activities designed to elicit and form consensus around emerging design principles for online and blended teacher PD programs in K-12 STEM education. The resulting principles are organized and presented around three themes: (1) Motivating and Sustaining Engagement that Builds Knowledge and Advances Professional Goals; (2) Creating Opportunities for Teachers to Collaborate as Learners; and (3) Supporting Reflection on Content and Practice. Participating awardees also described opportunities and challenges associated with each theme in online and blended settings. They highlighted opportunities for further research that could enhance the design principles presented in this report, inform the development of new principles, and address challenges that are common across projects. As these projects and other research efforts continue to generate empirical evidence around design approaches -- including the contexts in which and the audiences for whom they are effective, and their impacts on instruction and student learning -- the design principles presented in this report are intended to serve as guidance for the ongoing and future work of (1) researchers and developers of teacher PD programs and (2) administrators and teacher leaders who plan, implement, and facilitate online offerings for PD programming in their schools and districts. The emerging principles in this report were generated based on experience and initial evidence from across DRK-12 projects.