Substantive due process is the notion that due process not only protects certain legal procedures, but also protects certain rights unrelated to procedure.
Many legal scholars argue that the words “due process” suggest a concern with procedure rather than substance. Justice Clarence Thomas, one of this theory's most well-noted supporters, argued this point when he wrote that "the Fourteenth Amendment’s Due Process Clause is not a secret repository of substantive guarantees against unfairness"--understand the Due Process Clause. However, others believe that the Due Process Clause does include protections of substantive due process--such as Justice Stephen J. Field, who, in a dissenting opinion to the Slaughterhouse Cases wrote that "the Due Process Clause protected individuals from state legislation that infringed upon their “privileges and immunities” under the federal Constitution. Field’s dissenting opinion is often seen as an important step toward the modern doctrine of substantive due process, a theory that the Court has developed to defend rights that are not mentioned in the Constitution."
Substantive due process has been interpreted to include things such as the right to work in an ordinary kind of job, marry, and to raise one's children as a parent. In Lochner v New York (1905), the Supreme Court found unconstitutional a New York law regulating the working hours of bakers, ruling that the public benefit of the law was not enough to justify the substantive due process right of the bakers to work under their own terms. Substantive due process is still invoked in cases today, but not without criticism (See this Stanford Law Review article to see substantive due process as applied to contemporary issues).