466 U.S. 668 (1984)
Strickland v. Washington is the cornerstone of modern Sixth Amendment doctrine on the right to effective assistance of counsel.
What is the proper standard for adjudicating Sixth Amendment ineffective-assistance-of-counsel claims, and, under that standard, did Washington demonstrate that his lawyer's performance at capital sentencing was constitutionally deficient and prejudicial?
A defendant claiming ineffective assistance of counsel must show both: (1) Deficient performance—counsel's representation fell below an objective standard of reasonableness under prevailing professional norms, with courts applying a highly deferential review and indulging a strong presumption that counsel's conduct was reasonable strategic choice; and (2) Prejudice—a reasonable probability that, but for counsel's unprofessional errors, the result of the proceeding would have been different; a reasonable probability is a probability sufficient to undermine confidence in the outcome. Courts may resolve an ineffectiveness claim on the prejudice ground without addressing performance. Strategic choices made after thorough investigation are virtually unchallengeable, and choices made after less than complete investigation are reasonable to the extent they are supported by reasonable professional judgments. In capital sentencing, prejudice asks whether absent counsel's errors there is a reasonable probability the sentencer would have concluded that death was not warranted.
The Supreme Court adopted the two-prong deficiency-and-prejudice standard for ineffective-assistance claims and held that Washington failed to meet it. His counsel's decisions were within the wide range of professionally reasonable judgment, and, in any event, Washington did not show a reasonable probability that additional investigation or mitigating evidence would have changed the death sentence. The Court reversed the court of appeals and denied habeas relief.
Strickland supplies the ubiquitous framework for ineffective-assistance claims in criminal cases. It governs trial, sentencing, plea bargaining, and appellate representation, and its concepts—objective reasonableness and reasonable probability—anchor both classroom analysis and litigation strategy. The decision powerfully influences defense practice: attorneys must conduct reasonable investigations into facts and mitigation, document strategic choices, and advise clients competently, particularly in capital cases. For habeas review, Strickland's deference to counsel often combines with statutory deference to state-court decisions, creating the so-called "double deference" hurdle for petitioners. Later cases (e.g., Hill v. Lockhart on guilty pleas; Wiggins v. Smith and Rompilla v. Beard on mitigation duties; Lafler v. Cooper and Missouri v. Frye on plea negotiations; Harrington v. Richter on habeas deference) elaborate but do not displace Strickland's core test.