Levels of Scrutiny for Constitutional liberties

The Supreme Court uses several different "levels of scrutiny" to analyze whether individual liberties have been violated. The level of scrutiny used is determined by how strong of a liberty interest is at stake. The levels of scrutiny are the Rational basis test, intermediate scrutiny, strict scrutiny and the Undue burden test.

Rational Basis Test
The rational basis test states that a law will be upheld if it is rationally related to a legitimate government purpose. The challenger, the person complaining of a right's violation, in such a case has the burden of proof to show either that there is no conceivable legitimate purpose or that the law is not rationally related to it. Thus, the law's actual purpose does not need to be legitimate so last as  there is some possible legitimate use for the law. The government will almost always win such a case.

The rational basis test applies to non-fundamental rights such as:
right to practice a trade or profession, the right to physician-assisted death (this right has been outright denied), and a right to an education.

This test is used for laws affecting economic rights like the right to property.

Intermediate Scrutiny:
Under intermediate scrutiny a law will be upheld if it is substantially related to an important government purpose. The government has the burden of proof under this test. The actual purpose, a very good reason, not just some imagined one, must serve a legitimate government goal.

An example of cases that use intermediate scrutiny are the right to same-gender sexual relations. The Court in Lawrence v. Texas, which heard this issue, did not specifically say "intermediate scrutiny," but the explanation leads us to believe that's what they meant.

Strict Scrutiny
Under strict scrutiny a law will be upheld if the law is necessary to achieve a compelling government purpose. Here too, the government has the burden of proof. The Court will only apply the law's actual purpose and the law must have a strong purpose to carry out. The government usually loses these cases.

Strict scrutiny applies to rights considered to be fundamental such as:
right to marry, right to procreate, right to custody of children, right to keep the family together, right to control raising of children, right to purchase and use contraceptives (thank goodness), right to travel (covered under the Equal Protections Clause and Privileges and Immunities Clause), Right to Vote (EPC and P&I), Freedom of Speech (1st Amendment), Freedom of Association (1st), Free exercise of religion (1st, unless the law is one of neutral general applicability or if the Court doesn't approve of your religion).

The Undue Burden Test
This test was developed in Planned Parenthood v. Casey which changed the scrutiny for abortion to the undue burden test from strict scrutiny as decided in Roe v. Wade.

Under the undue burden test, prior to viability, states cannot prohibit abortions, but may regulate them so long as the regulations do not create an undue burden on the ability to obtain abortions.
This test is an enigma that only seems to apply to abortion cases.

Examples of Court decisions under the undue burden test:
A requirement for a 24-hour waiting period for abortions is not an undue burden.
A requirement that abortions be performed by licensed physicians is not an undue burden.
The prohibition of "partial birth abortions" is not an undue burden.