A LINQ (Language Integrated Query) developer uses LINQ framework in .NET for data manipulation. They write LINQ queries in C# or VB.NET to retrieve, manipulate, and update data from different types of sources like databases, XML documents, or in-memory objects. They ensure efficient data handling by using LINQ's capabilities like filtering, sorting, grouping, and joining. They also utilize LINQ's strong typing and IntelliSense to prevent errors. Furthermore, they may also be involved in debugging, testing, and optimizing LINQ queries, and integrating them into larger .NET applications to meet specific data-related requirements.