Mobile microlearning targets a new audience of learners: employees and workers outside of offices, using smartphones for flexible, anywhere, anytime training. The term 'mobile' emphasizes that the content is made for small screens of smartphones. According to literature and industry reports, micro-lessons are generally between 30 s and 5 min. While research shows that mobile microlearning is a promising approach, it remains unclear how the current systems have been 'built': What are the underlying principles of such platforms? The goal of this study was to explore mobile-microlearning platforms and to unpack their inherent design principles. We applied different methods: First, we reviewed literature in both academic publications and industry reports in two iterative rounds. Second, we conducted interviews with industry professionals, e.g., directors and entrepreneurs of mobile- and micro-learning systems. Results show a set of 15 principles regarding technical issues, pedagogical usability of micro-content interaction and sequenced instructional flow. They can be used to detect issues and challenges in existing mobile platforms and may inform meaningful design principles for future development. The results expose that a more critical eye on the learning design implied in the small-screen platforms is needed e.g., future systems may include learning designs for higher order thinking skills.