Potential host cities have to measure up against extensive criteria. Basically, the public authorities of a prospective city must be able to assure its respective National Olympic Committee (NOC) and the International Olympic Committee (IOC) that they'll do a top-notch job of organizing the games. This includes exhibiting suitable existing venues and unveiling design proposals for new ones. They must also present plans for transportation management, infrastructure improvements, accommodation strategies, technology systems, security policies and the torch relay. Then the IOC likes to test the waters by conducting an opinion poll, to get a feel for how excited the population is about possibly hosting the games.
If a city is in a spot prone to natural disasters, that can be an issue, as can the local weather. Obviously most cities would be more suitable candidates for the winter or the summer games, but even then, temperature extremes and climate play into consideration. Some prospective cities have to work around those issues. Temperatures in Doha, the capital of Qatar, can soar above 100 degrees Fahrenheit (38 degrees Celsius) during the summer -- which could affect athletes' performance. To sidestep the issue, in its bid for the 2020 games, the Qatar contingent suggested that the year's summer games be held in October and November, when temperatures in the area are more moderate.
Bidding cities also can help their chances by pledging to increase the participation in and practice of sports in general. For examples, representatives for Doha said that they would build a center to help increase women's participation in sports across the Middle East.
After the IOC carefully evaluates all of these considerations, the committee chooses one fortunate city to host the upcoming Olympic Games. Those not chosen, however, aren't completely out of luck. They can fine-tune their plan and bid again for another set of games. Often, it can take a time or two (or more) before a city's plan meets all of the IOC's expectations and gets voted as the winner.