With supply valves, you can turn off the water to a single fixture while fixing or replacing that faucet. If your plumbing system lacks supply stops, install them when you shut off the main valve ...

Causes: Worn out valves, seats and gaskets are common culprits. "Turning faucets on and off over the years wears these parts out," Hogan says. A faucet valve cartridge can also be to blame.