"To secure the blessings of liberty," the Preamble to the US Constitution proclaims, "We the People . . . ordain and establish this Constitution." The Constitution is said to secure liberty through three principal strategies: the design of the Constitution as a whole; structural arrangements, most notably separation of powers and federalism; and protection of rights. This chapter focuses on this third strategy of protecting liberty, in particular, through the Fifth and Fourteenth Amendments. We first examine the several approaches taken to the "Incorporation" of certain basic liberties "enumerated" in the Bill of Rights to apply to the state governments. We then examine the protection of "unenumerated" substantive fundamental rights or liberties against encroachment by the state governments or the federal government. We distinguish three phases in judicial protection of fundamental rights or liberties: (1) from 1887 to 1937; (2) from 1937 to 1973; and (3) from 1973 to the present. We discuss the substantive liberties that the Court has recognized under the categories of privacy, autonomy, or substantive due process. On one view, this list is a subjective, lawless product of judicial fiat and the whole enterprise is indefensibly indeterminate and irredeemably undemocratic. The other view, which we defend, is that the list represents a "rational continuum" of basic liberties stemming from "the individual’s right to make certain unusually important decisions that will affect his own, or his family’s, destiny." Tracing the due process inquiry from Roe (1973) to Lawrence v. Texas (2003) reveals how the Court and individual justices have waged a contentious battle among three available conceptions of what constitutes a tradition: abstract aspirational principles, concrete historical practices, and a "rational continuum" or evolving consensus. Lawrence signaled a return to a conception of tradition as a rational continuum or evolving consensus of aspirational principles and, in 2013, the Court drew upon Lawrence’s understanding of liberty together with Romer v. Evans’s understanding of equality in United States v. Windsor, striking down Section 3 of the Defense of Marriage Act. This chapter also discusses the so-called double standard concerning judicial protection of economic liberties as distinguished from personal liberties, sketching several theories’ views regarding Lochner v. New York in relation to Roe. Another issue is the stringency of the protection of liberties under the Due Process Clauses. Rather than two rigidly-policed tiers of scrutiny, with strict scrutiny automatically invalidating laws and deferential rational basis scrutiny automatically upholding them. the leading due process cases protecting liberty and autonomy – from Meyer (1923) through Lawrence (2003) – map onto a continuum of ordered liberty, with several intermediate levels of review. The chapter concludes by addressing substantive and institutional criticisms of constitutional protection of liberty.