Separation of powers under the United States Constitution
Courts check both the executive branch and the legislative branch through judicial review. This concept is not written into the Constitution, but was envisioned by many of the Constitution's Framers (for example, The Federalist Papers mention it). The Supreme Court established a precedent for judicial review in Marbury v. Madison. There were protests by some at this decision, born chiefly of political expediency, but political realities in the particular case paradoxically restrained opposing views from asserting themselves. For this reason, precedent alone established the principle that a court may strike down a law it deems unconstitutional.

Q: What is the court case that established judicial review?
Marbury v. Madison