The field of mobile health (mHealth), involving the use of smartphones, tablets, and other wireless devices to collect, aggregate, or disseminate health information, has been heralded as a health care revolution. Applications (apps) and sensors—many included in consumer-focused wearables, and others more specifically targeting diseases or therapeutics—present an array of potential tools and opportunities for people, patients, and physicians to monitor and improve health. For example, apps have the potential to help patients and physicians remotely diagnose acute conditions, like bacterial pharyngitis or otitis media. Remote sensor devices may assist in monitoring chronic conditions, like the blood glucose levels of patients with diabetes, or blood pressure in patients with hypertension. Smartphone-delivered messages and interventions may improve adherence to medication, exercise, or diet. However, in this issue of JAMA Internal Medicine, 2 articles describe core challenges to the development of reliable mHealth apps. Plante et al1 find that, while on the market, one of the most popular smartphone apps for measuring blood pressure was inaccurate. Murakami et al2 provide a similarly sobering look at the variation in accuracy among wearable devices designed to measure energy expenditure.