A Message Passing Interface (MPI) developer designs, implements, and optimizes parallel computing architectures for various applications. They use MPI, a standardized and portable message-passing system, to facilitate communication between different processes running concurrently on a distributed memory system. This involves writing, debugging, and testing MPI programs, ensuring efficient data exchange and synchronization between processes. The developer often works on complex computational problems, requiring a strong understanding of parallel computing concepts and algorithms. They also troubleshoot performance issues and may collaborate with other software developers to integrate MPI within larger systems or applications.