Constitutional basis 
This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding.
As the Supreme Court stated in Altria Group v. Good, 555 U.S. 70 (2008), a federal law that conflicts with a state law will trump, or "preempt", that state law:
Consistent with that command, we have long recognized that state laws that conflict with federal law are “without effect.” Maryland v. Louisiana, 451 U. S. 725, 746 (1981)
Intent of Congress presumed to be deference to states 
In Altria Group v. Good, the Court wrote:
When the text of a pre-emption clause is susceptible of more than one plausible reading, courts ordinarily “accept the reading that disfavors pre-emption.” Bates v. Dow Agrosciences LLC, 544 U. S. 431, 449 (2005).
In Wyeth v. Levine (2009), the Court emphasized what it called the "two cornerstones" of pre-emption jurisprudence:
First, “the purpose of Congress is the ultimate touchstone in every pre-emption case.” Medtronic, Inc. v. Lohr, 518 U. S. 470, 485 (1996) (internal quotation marks omitted); see Retail Clerks v. Schermerhorn, 375 U. S. 96, 103 (1963). [Medtronic: "[O]ur analysis of the scope of the statute's pre-emption is guided by our oft-repeated comment, initially made in Retail Clerks v. Schermerhorn, 375 U.S. 96, 103, ... (1963), that "the purpose of Congress is the ultimate touch-stone" in every pre-emption case."] Second, “[i]n all pre-emption cases, and particularly in those in which Congress has ‘legislated … in a field which the States have traditionally occupied,’ … we ‘start with the assumption that the historic police powers of the States were not to be superseded by the Federal Act unless that was the clear and manifest purpose of Congress.’ ” Lohr, 518 U. S., at 485 (quoting Rice v. Santa Fe Elevator Corp., 331 U. S. 218, 230 (1947) ).
See also Reilly, 533 U. S., at 541–542 (citation omitted):
Because ‘federal law is said to bar state action in [a] fiel[d] of traditional state regulation,’ namely, advertising, we ‘wor[k] on the assumption that the historic police powers of the States [a]re not to be superseded by the Federal Act unless that [is] the clear and manifest purpose of Congress’.
Federal Agency Administration Guiding Principles 
Executive Order 13132 of August 4, 1999 - See 64 Fed. Reg. 43, 255 - August 10, 1999, Sec. 4. Special Requirements for Preemption.
Agencies, in taking action that preempts State law, shall act in strict accordance with governing law.
(a) Agencies shall construe, in regulations and otherwise, a Federal statute to preempt State law only where the statute contains an express preemption provision or there is some other clear evidence that the Congress intended preemption of State law, or where the exercise of State authority conflicts with the exercise of Federal authority under the Federal statute.
(b) Where a Federal statute does not preempt State law (as addressed in subsection (a) of this section), agencies shall construe any authorization in the statute for the issuance of regulations as authorizing preemption of State law by rulemaking only when the exercise of State authority directly conflicts with the exercise of Federal authority under the Federal statute or there is clear evidence to conclude that the Congress intended the agency to have the authority to preempt State law.
(c) Any regulatory preemption of State law shall be restricted to the minimum level necessary to achieve the objectives of the statute pursuant to which the regulations are promulgated.
(d) When an agency foresees the possibility of a conflict between State law and Federally protected interests within its area of regulatory responsibility, the agency shall consult, to the extent practicable, with appropriate State and local officials in an effort to avoid such a conflict.
(e) When an agency proposes to act through adjudication or rulemaking to preempt State law, the agency shall provide all affected State and local officials notice and an opportunity for appropriate participation in the proceedings.
Evidence of Congressional intent to preempt 
In Altria Group v. Good, the Court reiterates that "Congress may indicate pre-emptive intent" in two ways: "through a statute’s express language or through its structure and purpose. See Jones v. Rath Packing Co., 430 U. S. 519, 525 (1977)".
Express Preemption 
Express preemption occurs only when a federal statute explicitly confirms Congress's intention to preempt state law. v. General Elec. Co., 496 U.S. 72 (1990). "If a federal law contains an express pre-emption clause, it does not immediately end the inquiry because the question of the substance and scope of Congress’ displacement of state law still remains." Altria Group v. Good
Implied Preemption 
Implied preemption can occur in two ways: field preemption or conflict preemption. Massachusetts Ass'n of HMOs v. Ruthardt, 194 F.3d 176, 179 (1st Cir. 1999).
1. Conflict preemption 
Under the Supremacy Clause, any state law that conflicts with a federal law is preempted. Gibbons v. Ogden, 22 U.S. 1 (1824). Conflict arises when it is impossible to comply with both the state and federal regulations, or when the state law interposes an obstacle to the achievement of Congress's discernible objectives. Gade v. National Solid Wastes Mgmt. Ass'n, 505 U.S. 88, 98 (1992).
- Actual conflict. A conflict exists if a party cannot comply with both state law and federal law (for example, if state law forbids something that federal law requires). Florida Lime & Avocado Growers, Inc. v. Paul, 373 U.S. 132, 142-43 (1963).
- Obstacle. In addition, even in the absence of a direct conflict between state and federal law, a conflict exists if the state law is an obstacle to the accomplishment and execution of the full purposes and objectives of Congress. Crosby v. Nat’l Foreign Trade Council, 530 U.S. 363, 372-73 (2000).
- Minimum Safety Standard vs. Uniform Safety Standard
- Often there may be a question of frustration of congressional purpose or the state law standing as an obstacle to congressional intent. This will raise a question of whether congressional or administrative intent in passing the law was uniformity or minimum national safety standards. Congressional intent may be to allow States to pass laws that will “establish greater safety than the minimum safety achieved by a federal regulation intended to provide a floor.” Geier v. American Honda Motor Co., 529 U.S. 861, 870, 120 S. Ct. 1913, 146 L. Ed. 2d 914 (2000).
- Alternatively, the purpose of a federal law could be to set a uniform national standard. This was the case in Geier, where the National Traffic and Motor Vehicle Safety Act of 1966 required auto manufacturers to equip a certain number of their 1987 vehicles with passive restraints. Id. at 864-865. the question before the Supreme Court was whether the Act pre-empted state common-law tort claims saying that the auto manufacturer, although in compliance with the Act, “should nonetheless have equipped a 1987 automobile with airbags.” The court indicated that, despite a savings clause, the statute “reflects a desire to subject the industry to a single, uniform set of federal safety standards. Its pre-emption of all state standards, even those that might stand in harmony with federal law, suggests an intent to avoid conflict, uncertainty, cost, and occasional risk to safety itself that too many different safety–standard cooks might otherwise create.” Id. at 871.
2. Field preemption 
Even without a conflict between federal and state law or an express provision for preemption, the courts will infer an intention to preempt state law if the federal regulatory scheme is so pervasive as to “occupy the field” in that area of the law, i.e. to warrant an inference that Congress did not intend the states to supplement it. Gade v. National Solid Wastes Mgmt. Ass'n, 505 U.S. 88, 98 (1992). See also Rice v. Santa Fe Elevator Corp. For example, the courts have held that the National Labor Relations Act (NLRA) preempts state laws directed at conduct actually or arguably prohibited or protected by the NLRA or conduct Congress intended to leave unregulated. San Diego Bldg. Trades Council v. Garmon, 359 U.S. 236, 244 (1959); Machinists v. Wisconsin Emp. Rel. Commission, 427 U.S. 132, 140-48 (1976).
- California Fed. Sav. & Loan Ass'n v. Guerra, 479 U.S. 272 (1987) (reviewing preemption doctrine).
- Buzbee, William W. (2009). Preemption choice. Cambridge University Press. ISBN 978-0-521-88805-9.
- "Federal Preemption: The Power to Supersede State Regulation". Public health law and ethics. University of California Press. 2002. ISBN 978-0-520-23174-0.
- "Federal Preemption and the Role of State and Local Laws". Technology, Law, and the Working Environment. Island Press. 1996. ISBN 978-1-55963-446-5.
- Morrison, Alan B. (1998). "Preemption Controversies". Fundamentals of American law. Oxford University Press US. ISBN 978-0-19-876405-2.
- Lively, Donald E.; Weaver, Russell L. (2006). "Federal Preemption of State Power". Contemporary Supreme Court cases. Greenwood Publishing Group. ISBN 978-0-313-33514-3.
- Henkin, Louis (2002). "Federal Preemption". Foreign affairs and the United States Constitution. Oxford University Press. ISBN 978-0-19-826098-1.
- Drahozal, Christopher R. (2004). "Federal Preemption of State Law". The supremacy clause. Greenwood Publishing Group. ISBN 978-0-313-31447-6.
- Perry, Ronen (2011). "Differential Preemption" Ohio State Law Journal, Vol. 72 (discussing maritime preemption)