Researchers who work with stem cells have ambitious goals. Some want to cure cancer or treat heart disease. Others want to grow the tissues and organs that patients need for transplants. Some groups are even working to develop highly personalized medicines, tailored to an individual’s genetics. All of these ideas face a similar hurdle, however: The development of measurement tools for stem cell production is challenging, making it hard to determine what makes various new stem cell-related products safe, effective or high-quality.