Montesquieu's theory of separation of powers deals with the governance of a state. According to this theory, the state must be divided into separate branches with independent powers and duties. This type of division is beneficial for preventing overlapping of powers and duties of the different wings of the government, and for minimizing conflicts between them. In general, states are divided into three branches - legislature, executive, and judiciary. While the legislature is entrusted with the duty of making laws, the executive has to take administrative and policy decisions, and the judiciary has to interpret and apply the laws. This type of division prevents concentration of powers in a single branch or entity. Apart from dividing the powers, such a governance system must have proper checks and balances, so as to prevent any one of the branches from becoming the supreme power. In the United States, you may come across various constitutional and statutory provisions that act as checks and balances. While the legislature or Congress introduces bills, the executive (president) can veto the bills passed by Congress. The judiciary can exercise the power of judicial review and annul laws that are unconstitutional or inconsistent with a higher authority. However, the power of judicial review is not expressly provided by the U.S. Constitution, but is derived from Articles III and VI, as an implied power. In short, the constitution is the fundamental and supreme law of the country, and the judges are entrusted with the duty to interpret and apply the constitutional provisions. If a federal or state statute conflicts with the constitution, the latter shall prevail, and the former has to be treated as unenforceable. The definition of judicial review is as follows: "A court's authority to examine an executive or legislative act and to invalidate that act if it is contrary to constitutional principles." The term also refers to one court's review of another court's proceedings and judgments, especially, a higher court review of a lower court. It can also be a review of an administrative body's factual or legal findings. Given below are a few examples or court decisions where the doctrine of judicial review was applied, leading to certain laws being held unconstitutional. Marbury v. Madison, 5 U.S. 137 (1803) is the first and foremost case in which the Supreme Court exercised its power of judicial review. However, this does not mean that the doctrine was not applied by the judiciary before this case. In fact, state courts had been exercising the power of judicial review much before the Constitutional Convention in 1787. Various state laws were struck down, as they were found to be unconstitutional or inconsistent with higher laws. In this case, William Marbury petitioned the Supreme Court for a writ of mandamus, seeking delivery of his commission from the then Secretary of State James Madison. Marbury was appointed as the Justice of Peace in the District of Columbia by President John Adams, a few days before the latter left office. However, the new President Thomas Jefferson ordered James Madison not to deliver the documents. William Marbury filed the petition before the Supreme Court, according to the provisions of the Judiciary Act of 1789. This statute enabled the Supreme Court to issue some types of writs, but such powers were not granted by the constitution. Though the court held that Marbury had the right to get his job, section 13 (which authorized the court to issue certain types of writs) of the Judiciary Act of 1789 was held unconstitutional. It was ruled that the court lacked jurisdiction to issue a writ of mandamus, as per the constitution. So, the court exercised the power of judicial review to strike down section 13 of the Judiciary Act of 1789. This is one of the most controversial decisions made by the Supreme Court of the United States, as it ruled that African-Americans cannot be American citizens, and have no right to sue in federal courts. However, the decision is highly significant, as the court exercised the power of judicial review by ruling that the Missouri Compromise was unconstitutional, and that Congress did not have the authority to prohibit slavery in the territories. This is one of the classic examples in which the Supreme Court exercised the power of judicial review by striking down certain state laws as unconstitutional. In this case, the court struck down state laws that allowed setting up of separate schools for students based on their color. It was held that 'Separate educational facilities are inherently unequal', and hence, unconstitutional. On December 8, 1990, Margaret P. Gilleo placed a sign in the front yard of her home located in Ladue, Missouri. It read, "Say No to War in the Persian Gulf, Call Congress Now". The sign disappeared twice, and she lodged a police complaint. She was told that such signs were illegal according to the city ordinance. She filed a suit against the City Council in the federal court, which struck down the ordinance as unconstitutional. On appeal, the Supreme Court upheld the decision of the lower court, and ruled that the ordinance provisions against yard signs violated the First Amendment of the Constitution. According to the Supreme Court decision in consolidated cases Arizona Free Enterprise Club's Freedom Club PAC v. Bennett and McComish v. Bennett (Nos. 10-238 and 10-239), provisions for public funding of elections in The Arizona Citizens Clean Elections Act were held unconstitutional. It was ruled that such funding will discourage candidates who chose to raise private donations from raising or spending campaign funds, and thus discourage political debate. The majority view was that the Act violated the First Amendment rights of candidates who raise private money for campaigning. In short, the doctrine of judicial review is based on the concept that the constitution is the supreme law of the land. No one is above the law, not even the executive (President) and the legislature (Congress). The Supreme Court can exercise the power of judicial review to overturn federal and state legislation, as well as other governmental actions that are inconsistent with the constitution. Judges of all courts (both federal and state) can exercise this power while deciding cases brought before them in a court of law.