Courts uphold constitutionality in legislation through the process of judicial review, where they assess laws against constitutional standards. This involves interpreting the supreme law and ensuring that legislative acts do not infringe upon individual rights or exceed the powers granted to government entities. By striking down unconstitutional laws, courts protect democratic principles and maintain a system of checks and balances. Judicial review serves as a mechanism for ensuring that the rule of law prevails in governance, fostering accountability and legitimacy within the legal framework of a society.