The author traces the growth of K-12 online learning in the United States from its modest genesis in the mid-1990s with 50,000 students to the more than 4 million enrollments today, the fastest scaling ever of any innovation in K-12 education. The evolution from one-size-fits-all online courses to innovative, blended instructional models that are student-centered, highly personalized, competency-based, and self-paced is an important trend in new blended learning models emerging among budgetary constraints. Significant barriers to adoption remain in many school districts. (Contains 1 figure.)