In An Architectural Theory of Relations: Sigfried Giedion and Team X, Sarah Deyong uncovers an important aspect of the theoretical framework underpinning Team X’s work: Sigfried Giedion’s philosophical orientation and aesthetic theories in the late 1940s and early 1950s. Because Giedion is often seen as an old-guard CIAM (Congrès Internationaux d’Architecture Moderne) member that Team X opposed, his contribution to Team X’s design method has been grossly underestimated. Deyong rectifies the link between Giedion and Team X to accomplish two goals. She casts new light on Giedion’s unique contribution to the reinvention of modern architecture at midcentury, and she offers a new interpretation of Team X’s legacy, constructed around missing pieces of the group’s intellectual history. Deyong’s evidence for this argument derives from two archival sources in particular: Giedion’s papers in the Archives of the Institute for the History and Theory of Architecture (gta Archives) in Zurich, and the unpublished transcripts of Team X meetings, housed in the Collection Het Nieuwe Instituut in Rotterdam.