Programs + Communication Structures = Systems Abstract: The title of Niklaus Wirth's classic 1976 textbook, "Algorithms + Data Structures = Programs", captured the essence of software design in the era of sequential computing. Now, however, we need a new slogan for the era of communication-intensive distributed computing. I will describe the use of session types to specify and verify the communication protocols that form the structure of concurrent and distributed systems. Ultimately session types should form the basis for new approaches to the design and engineering of large-scale communication-oriented systems.