William Joseph Brennan, Jr. (April 25, 1906 – July 24, 1997) was an Associate Justice of the Supreme Court of the United States. Serving from October 16, 1956 - July 20, 1990, he was known for his liberal opinions, which reportedly included the per curiam opinion in Cooper v. Aaron that established judicial supremacy.
Justice Brennan ostensibly filled a vacancy left by Sherman Minton, but more accurately filled a "Irish-Catholic" seat vacated by Frank Murphy in 1949. President Dwight Eisenhower, concerned about his reelection only about a month later, hoped to appeal to northeastern Democratic voters in picking Brennan.
Justice Brennan wrote three of the most liberal court decisions in history:
- This was officially issued per curiam, but authorship has been informally credited to Justice Brennan. 
- "He-not the insecure, less gifted Justice Blackmun-was the driving force behind the final version of Roe v. Wade, which Blackmun originally had planned as a cautious, narrow decision. A little-noted event reveals the extent of Brennan's role. Brennan's law clerks included Roe and its companion case, Doe v. Bolton, in the bound volume of Justice Brennan's decisions of the term, along with a footnote stating: 'These cases are included with Justice Brennan's opinions for the October term 1972 because the opinions for the Court were substantially revised in response to suggestions made by Justice Brennan.' It was Justice Brennan who was largely responsible for the expansion of the abortion regime outlined in Roe into a 'fundamental' right to abortion on demand that was used during his tenure to strike down parental consent and notification laws, viability testing, informed consent requirements and numerous other legislative measures."