A MapReduce developer is responsible for the design and development of big data applications using the MapReduce programming model. They handle large datasets, convert complex processing instructions into simpler tasks, and distribute them across a cluster of machines. These developers write codes for Map (which performs filtering and sorting) and Reduce (which performs summary operations) functions. They also manage data transfer between these functions. They ensure data integrity, troubleshoot issues, optimize system performance, and often work with other big data technologies like Hadoop or Spark. Understanding of parallel computing, algorithms, and data structures is crucial for this role.