Do planes have keys? One thing I've always wondered is: do airplanes have keys that you put in the ignition to start?

And on the outside doors? Otherwise how are they secured and started?

I'm interested to know about a broad range of aircraft; from GA, all the way to airliners.
Small planes tend to have keys, but not all.  The bigger the plane the less likely a key is needed and once you get into jets, I can't think of any that need keys for engine starts.  The starting and securing of engines with no keys comes down to switches and knobs.  

Likewise with exterior doors, on smaller planes including private end executive jets, you can find keyed exterior doors and luggage compartments.  With transport jets at airlines you won't find keys. To secure those doors the jet bridge is pulled away from the airplane so there is no physical access to the boarding door.