Dan Ingalls was the main implementor of the Smalltalk-80 system and its precursors, Smalltalk-72, Smalltalk-76, and Smalltalk-78. He is responsible for much of the detailed design of the Smalltalk language and system, as well as the formulation of key object-oriented design principles. Dan also invented BitBlt, a general-purpose graphical operation, and pop up menus, making window systems and GUIs a reality. He made the unique Smalltalk environment what it is, with its support for pure object-orientation, reflection, live programming, and persistence.

Later, Dan led the development of Squeak, a practical and very compact Smalltalk system with a virtual machine written entirely in Smalltalk. Squeak and its offshoots like Pharo continue to have a thriving open-source community. Based on Squeak, Dan worked on several other pioneering live programming systems such as the Lively Kernel for web programming, and EToys, an educational programming environment. EToys was the direct forerunner of Scratch, with its huge influence on education and programming for children

Dan’s work has had enormous impact, foreshadowing modern computing in many ways.