engine light on 2003 BMW 325i

engine service light is on, what could the most basic problem be?

2 answers
Check out the RepairPal article on the Check Engine Light - http://repairpal.com/check-engine-light and the page for your car - http://repairpal.com/bmw-325i-2003

That should get you started. The light indicates that there is a failure in your emission control system, and it could be anything from a loose gas cap to a failing sensor.
The most basic thing could be a loose or bad fuel cap. Once I replaced mine, it went away.

Another could be a miss-fire. How long has it been since you changed plugs? Did you use the best plugs?