Accessibility is not a feature to add at the end — it is a fundamental design principle that improves products for all users. This article covers the business case for accessibility (legal compliance, market expansion, improved usability), the core principles of WCAG (perceivable, operable, understandable, robust), and practical implementation guidance for web and mobile products. It examines how solutions designed for users with disabilities — curb cuts, closed captions, voice interfaces — became beloved by all users. The article includes an accessibility audit checklist and resources for building accessibility into design and development workflows.