A running toilet is typically caused by a malfunction in one or more of its internal components. The most common culprits are a faulty flapper that doesn't seal properly, allowing water to continuously leak from the tank to the bowl; a malfunctioning fill valve that fails to shut off the water supply when the tank is full; a tangled or loose chain connecting the flush handle to the flapper, preventing it from sealing; or a sticking flush handle that doesn't return to its proper position after flushing. In some cases, an excessively high water level in the tank can also lead to continuous running as water spills into the overflow tube. Diagnosing and repairing the specific issue can usually stop the toilet from running and wasting water.