Systems thinkers see the world as a series of 'systems', cohesive, interconnected entities bounded by space and time, defined by their structure and purpose, and influenced by their wider environment. Systems thinkers hold that whether natural or human made, systems share properties and dynamics that allow us to compare diverse phenomena and grasp real world complexity. Social innovation scholars use systems thinking to understand how new social processes can shift dynamics within the systems they are introduced, or shift the systems themselves, leading to wider transformation and addressing grand challenges that we face. We discuss systems, systems thinking, how systems thinking has shaped social innovation scholarship, and criticisms that aim to hold systems thinking accountable to its lofty claims of holism.