In my experience, for all practical purposes, if you flight starts or ends in controlled airspace, you need to file a flight plan beforehand. If you're just looking for a transit, you can usually file via the radio while in the air.
However each country has specific rules, and these rules may be superseded by the regulations of individual airports. The only real definitive answer will be in the AIP.
Last edited by BackPacker; 30th April 2015 at 11:00.