A design system is a shared library of components, tokens, and documented patterns that your design and engineering teams both work from. You need one the moment you have more than one designer, more than one engineer touching the UI, or more than one product surface. Without it, every new feature introduces inconsistency, every designer has a different interpretation of your brand, and every engineer re-solves the same component problems independently. The hidden cost compounds fast. We have seen teams where 30 percent of engineering time was going to UI inconsistency rework before they had a design system.