CSS-in-JS is a styling approach that allows developers to write CSS directly within their JavaScript code. This method enables styles to be scoped to specific components, making it easier to manage and maintain styles in large applications. It often leverages JavaScript's dynamic capabilities, allowing for conditional styling and theming. Popular libraries like Styled Components and Emotion facilitate CSS-in-JS by providing tools to create styled components with a clean syntax. This approach promotes better organization of code and can improve performance by reducing the number of style sheets loaded in a project.