Atoms are the smallest UI pieces: button, input, label, icon. Molecules combine atoms: search bar, form field with label. Organisms are larger blocks: header, product card, checkout form. Templates and pages compose organisms into full layouts.

Small, focused components are easier to reuse and test. Compose by passing props and children; avoid deep nesting of logic. When the same pattern appears in multiple places, extract a component.

A design system is a shared set of components and tokens (colors, spacing, typography) that all products use. You implement the components once; teams consume them so UIs look and behave consistently.

Tokens (e.g. CSS variables or a theme object) keep colors and spacing consistent. Document usage (when to use which component, dos and don’ts) so the system scales with the team. One source of truth reduces drift and duplicate implementations.

When many teams own different parts of the product, micro-frontends let each team ship its part as an independent app; a shell composes them at runtime. Components scale to the app level: ownership, deploy, and tech stack can differ per team.

[Building Micro-Frontends](https://www.oreilly.com/library/view/building-micro-frontends/9781492082996/) (Luca Mezzalira, O'Reilly) extends the component mindset to the application level.