The concept of complexity is one of the most fundamental of big history fundamentals. The concept of complexity has great potential for understanding the shared qualities of otherwise disparate systems, explaining large-scale change, and comparing different types of complex systems, including human societies. Given this potential, it seems extraordinary that the concept has not penetrated the academic zeitgeist more thoroughly. I argue that four key roadblocks are holding the concept of complexity, and by extension, big history, from broader acceptance in the academy: first, the term “complexity” in its technical usage is not intuitive to people outside the fields of big history and complexity science; second, there is a lack of consensus even among big history scholars on the definition of complexity; third, measuring large-scale change over thousands, millions, or billions of years may lead to imprecision and oversimplification; and fourth, complexity, while an objective indicator of change, is closely tied to contested, subjective, culturally-specific notions of human progress. This paper argues that the concept of complexity, despite these roadblocks, has significant utility in fields that consider large-scale change. Ultimately, this paper aims to provide more clarity and precision around the concept of complexity to strengthen one of the key foundations of big history.