Christopher Alexander, who died in March 2022, was undeniably one of the most influential, if sometimes controversial, urban thinkers of the last half‐century. From Notes on the Synthesis of Form, his first book and Harvard PhD thesis, to the landmark “A City is Not a Tree,” to the classic best‐sellers A Pattern Language and The Timeless Way of Building, to his more difficult and controversial magnum opus, The Nature of Order, Alexander has left a body of work whose breadth and depth is only now coming into view. Yet Alexander’s legacy is also the subject of intense debate and critique within the planning and design fields. This introduction provides an overview of the thematic issue of Urban Planning titled “Assessing the Complex Contributions of Christopher Alexander.” Its purpose is to provide greater clarity on where Alexander’s contribu-tion is substantial, and where there are documented gaps and remaining challenges. Most importantly, the thematic issue aims to identify fruitful avenues for further research and development, taking forward some of the more promising but undeveloped insights of this seminal 20th‐century thinker.