The ABA Guidelines for the Appointment and Performance of Defense Counsel in Death Penalty Cases (“Guidelines”), as revised in 2003, continue to stand as the single most authoritative summary of the prevailing professional norms in the realm of capital defense practice. Hundreds of court opinions have cited the Guidelines. They have been particularly useful in helping courts to assess the investigation and presentation of mitigating evidence in death penalty cases. This Article will discuss how these Guidelines have come to reflect prevailing professional norms in this critical area of capital defense practice and how that practice has developed in the era of the modern U.S. death penalty. One of the principal arguments we will make in this Article is that courts interpreting the Sixth Amendment’s guarantee of effective assistance of counsel should look to what competent lawyers ought to do rather than what some lawyers appointed to represent capital defendants actually do . . . .