If you hear designers toss out this term, they are likely referring to Bloom’s Two-Sigma Problem: the research finding that one-on-one instruction (direct tutoring) is as much as two standard deviations (sigmas) more effective than conventional group instruction. This raises significant challenges and opportunities related to personalized learning, particularly with the affordances of today’s educational technologies. For example, to what extent can a learning system function as a personal tutor? If a learning is engaging on their own with a self-paced course that is responsive to their needs and skill level, is this also as effective one-on-one tutoring with a human instructor? Can such systems be created and supported in a sustainable way? Some startups and design strategists are attempting to crack this nut as we speak, and the challenge holds great promise for the future of digital learning.