Accessibility in K-12 and higher education is becoming an increasingly complex terrain to traverse as schools increase online materials and instructional delivery options. This white paper provides an overview of critical terms, legal precedents, and other considerations for course designers, instructors, and administrators as they work to improve the educational experiences of learners with disabilities. Key questions around accessibility include the following: (1) How do national laws that govern disability access apply to online courses?; (2) What case law exists to guide online course design and delivery in various educational settings?; (3) What issues emerge regarding online course access that might be unique to higher education and to K-12 settings? What issues are shared?; and (4) What support do online course designers need in order to generate accessible courses for learners across the life span (from K-12 to higher education)? The first section of this white paper provides definitions for concepts related to accessibility in digital educational environments. Second, accessibility in online course design from both practical and policy perspectives are elaborated for both higher education and K-12 education. Helpful references and resources -- including active online communities of practice -- are provided at the end of the document.