U.S. in-service PK-12 teachers have less time for professional learning (PL) but their needs for applicable professional development (PD) have increased (Birch & Lewis, 2020) and teachers, both new to the field and veterans, are looking to reskill and upskill their knowledge and teaching abilities (Bayar, 2014; Birch & Lewis, 2020; Leung, 2021). Microlearning is an emerging strategy to support PK-12 in-service teacher PL (Birch & Lewis, 2020; Czyz, 2017; Kapp & Defelice, 2018), but few studies exist on how to design microlearning for PK-12 in-service teacher PD. Teachers are also turning to video-streaming social media sites such as YouTube and TikTok for professional learning, indicating they are learning more informally than from the formal professional learning provided at their work (Bal, 2023; Czyz, 2017). This self-directed learning, coupled with the short nature of videos on these sites (e.g., 10 minutes or less), suggests informal microlearning may be a solution to the PL needs of U.S. PK-12 in-service teachers. This study investigated the overlap of microlearning and informal learning for PK-12 in-service teacher PD, attempting to answer the research question, what design considerations are most important when developing informal microlearning for PK-12 in-service teacher professional development? A three-phase traditional, online Delphi method was utilized, engaging three panels of experts: (a) PK-12 in-service teacher PD experts, (b) microlearning experts, and (c) informal learning experts, through five surveys. The first, qualification, survey was sent to 295 experts from these three panels. Sixty (21.35%) of those identified experts completed the qualification survey with 54 experts qualifying for the study. Of these 54 experts, 38 (70.37%) consented to participate in the study with the Phase 1 survey, and 32 of these 38 (84.21%) experts completed the study. The surveys were analyzed both qualitatively and quantitatively through a multi-method format, engaging participants over 15 weeks. Phase 1 focused on the design considerations for PK-12 in-service teacher PD, microlearning, and informal learning, resulting in 75 design consideration phrases. In Phase 2, panels were separated with participants selecting the most important design considerations with consensus defined as more than 60% of participants. This created three distinct lists of design considerations, one for each expert panel. Phase 3, ranking of design considerations, occurred in two parts. Part 1 included the initial ranking, and Part 2 allowed participants to rerank the list if they felt it was needed, and to reach consensus. The final consensus within the panels was strong for the PK-12 teacher PD and microlearning panels and moderate for the informal learning panel. A between panel analysis was then conducted for Phases 2 and 3. Sixteen design considerations were selected by at least two panels and ranked, which was the focus of the discussion. All 16 of the final design considerations selected by at least two panels aligned with at least one research area, PK-12 in-service teacher PD, microlearning, and informal learning. Three design considerations aligned to all three research areas, which supports the limited studies available in informal microlearning design for PK-12 in-service teacher PD, and seven design considerations aligned solely to PK-12 in-service teacher PD research, suggesting there are specific design considerations needed for the PK-12 in-service teacher PD learner. The findings from this study support Zemke and Zemke's (1988) "eclectic" approach by overlapping the most important design considerations from three expert panels to create a synthesized list of 16 design considerations when creating informal microlearning for PK-12 in-service teacher PD. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]