A Backbone.Marionette developer builds and maintains complex web applications using the Backbone.Marionette framework. This involves designing scalable application architecture, implementing user interfaces, and ensuring seamless data flow between server-side and client-side components. They manage views and layouts, handle events, and ensure smooth navigation. They also test the application for performance and troubleshoot issues. Additionally, they often work with other technologies such as JavaScript, HTML, and CSS. They might also be responsible for improving user experience, integrating third-party APIs, and keeping up-to-date with latest industry trends to incorporate into their development process.