Not all engines use timing belts, making them an often-overlooked maintenance operation. But ignoring it could eventually turn an otherwise good day ugly, especially if you own an "interference engine" - so called because if the timing belt breaks, the valves will contact the cylinders, causing engine damage. Even with a non-interference engine, timing belt failure will leave you stranded, as the engine cannot run without it. Ideally, it should be replaced at regular intervals to prevent the added expense and inconvenience of an unplanned breakdown.
Doing the job right - which prevents premature future failure - includes replacing the water pump if driven by the timing belt, inspecting all tensioners, idler pulleys and oil seals and replacing any that are suspect, and making sure the seals around the belt cover are sound to prevent moisture and dirt from contaminating the belt. On the plus side, timing belts on modern vehicles are much more durable than in the past and can last up to 160,000 kilometres. Most manufacturers include timing belt replacement intervals in their maintenance schedule.