An Abstract Data Type (ADT) developer designs and implements data structures that organize, store, and manage data in a computer. They define the data type, the operations that can be performed on the data, and the relationship between the data and operations. They ensure encapsulation by hiding the implementation details from users, offering only the necessary interfaces. Their work involves designing algorithms and writing code for operations like insertion, deletion, searching, and sorting. They also ensure data integrity and consistency. They work in various programming languages like C++, Java, and Python, and their work is foundational to software development.