In this era of increasing ubiquity of sensing and computing devices, we still struggle mightily engaging people with systems designed to evaluate and improve health. Several fundamental challenges face developers, clinicians, researchers, and entrepreneurs building mobile applications in health as a result. Academic and industry success metrics are even at odds with designing for sustained engagement, instead prioritizing significant changes to behavior in the former and massive growth in the latter. I will present examples from a consumer health app, Wellcoin, and my own research that highlight successes and (more interestingly!) failures in engaging a wide variety of people. Preview: gamification stinks, great design is nowhere near as important as we like to believe, and no tactics are one size fits all.