A CNTK (Microsoft Cognitive Toolkit) developer designs, builds, and implements machine learning models using the CNTK framework. They utilize its deep learning capabilities for tasks like image, speech, and text recognition or prediction, among others. They work with neural networks, train models, test their efficiency, and optimize them for better performance. They may also contribute to improving the toolkit itself. Their job involves understanding complex mathematical algorithms, programming, data analysis, and problem-solving. They often collaborate with data scientists and other developers to create AI applications for various industries like healthcare, finance, or technology.