The mapping of the ancient city of Teotihuacan was an archaeological project of singular importance in the history of archaeology. In this paper, we discuss the origin and history of the Teotihuacan Mapping Project (TMP) through a series of personal vignettes written by the project's leader, René Millon, which are put into larger context by Jeff Altschul, one of the many students who worked on the project. We examine the characteristics that led to the TMP's successes and its shortcomings and discuss lessons learned that may be of value to planning future big, complex archaeological projects. We argue that above all, a big project needs a big problem to solve. In the case of the TMP, the problem was the origin of the city. Marshaling a team of diverse talents, Millon and his colleagues were able to make many key decisions in ways that successfully overcame problems that had not been heretofore confronted by archaeologists. These decisions include the use of low-altitude aerial photography, the definition of sites to include nonliving urban spaces, the sampling of surface artifacts, strategic test excavations, computerized data management and sophisticated statistical analyses, and a unique manner of publication. Less successful was the project's record in publishing descriptive data. The project's success lay in its ability to take on an important problem and to follow through, even though some tasks required decades to complete and others remain to be completed.