Given that we are halfway through the summer travel season and we are seeing packed flights and in many cases oversold flights. What happens when everyone shows up for the flight and there are more passengers than seats? Is it first come, first serve? Or do you have to take whatever the airline offers? Here is what the Department of Transportation has to say about your rights and what your options are under these circumstances. http://airconsumer.dot.gov/publications/flyrights.htm#overbooking. If you are in Europe or on a flight going to Europe or flying on a European carrier, then the European Union rules may apply. They can be found at: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32004R0261:EN:HTML:NOT. There are some significant differences between the US DoT and the EU regulations and how they impact you.
I would suggest it is worth one’s time to get to know these regulations as they may be the difference between getting where you want to go and sitting in the airport waiting.