Thelen's research was focused on human development, especially in the area of infant development.
Thelen is known for her works on infant development, particularly those that focused on complex movement and behavioral development. Thelen applied chaos theory to the research of how babies learn to walk and interact with the world around them. In Thelen's view behavior emerges as a pattern from all the streams that flow into the river of infant development. Or, as she wrote "The mind simply does not exist as something decoupled from the body and experience". She suggested that an infant already has basic motor patterns at birth as demonstrated by stepping reflex and spontaneous kicking.