This is the third edition of AISC Design Guide 1. The first edition was published in 1990 as Column Base Plates by J.T. DeWolf and D.T. Ricker. The second edition was published in 2006 as Base Plate and Anchor Rod Design by J.M. Fisher and L.A. Kloiber. This third edition incorporates and updates the content of the previous editions while also providing significant expansions in coverage related to base connection design. Significant expansions to this Design Guide include the addition of Chapter 3 addressing the relationship between the structure and base connections; the addition of Chapter 5 pertaining to embedded base connection design; the addition of Chapter 6, which focuses on seismic design of base connections; and the addition of Appendices C and D, which provide guidance regarding the simulation and representation of base connections. This edition also significantly expands upon the number of design examples.