A BEM (Block, Element, Modifier) developer designs and implements web applications using the BEM methodology, an approach to front-end development that helps create reusable, maintainable, and scalable code. They break down the user interface into separate components (blocks), parts of these components (elements), and states or variations of these components (modifiers) to simplify the coding process. They ensure each component is independent, reducing the risk of cascading changes. BEM developers typically collaborate with other developers, designers, and stakeholders, and they are responsible for debugging, testing, and maintaining the code.