522
Opinion of the Court
stances, applying a heavy measure of deference to counsel's judgments." Id., at 690-691.
Our opinion in Williams v. Taylor is illustrative of the proper application of these standards. In finding Williams' ineffectiveness claim meritorious, we applied Strickland and concluded that counsel's failure to uncover and present voluminous mitigating evidence at sentencing could not be justified as a tactical decision to focus on Williams' voluntary confessions, because counsel had not "fulfill[ed] their obligation to conduct a thorough investigation of the defendant's background." 529 U. S., at 396 (citing 1 ABA Standards for Criminal Justice 4-4.1, commentary, p. 4-55 (2d ed. 1980)). While Williams had not yet been decided at the time the Maryland Court of Appeals rendered the decision at issue in this case, cf. post, at 542 (Scalia, J., dissenting), Williams' case was before us on habeas review. Contrary to the dissent's contention, post, at 543, we therefore made no new law in resolving Williams' ineffectiveness claim. See Williams, 529 U. S., at 390 (noting that the merits of Williams' claim "are squarely governed by our holding in Strickland"); see also id., at 395 (noting that the trial court correctly applied both components of the Strickland standard to petitioner's claim and proceeding to discuss counsel's failure to investigate as a violation of Strickland's performance prong). In highlighting counsel's duty to investigate, and in referring to the ABA Standards for Criminal Justice as guides, we applied the same "clearly established" precedent of Strickland we apply today. Cf. 466 U. S., at 690-691 (establishing that "thorough investigation[s]" are "virtually unchallengeable" and underscoring that "counsel has a duty to make reasonable investigations"); see also id., at 688-689 ("Prevailing norms of practice as reflected in American Bar Association standards and the like . . . are guides to determining what is reasonable").
In light of these standards, our principal concern in deciding whether Schlaich and Nethercott exercised "reasonable
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