This article is currently lacking one or more significant sections; these sections may need further research or thought and are expected to be completed later as time permits.
The United States in particular, and the global economy in general, depends heavily on non-renewable fossil oil as a fuel for transportation. More specifically, it depends upon oil remaining relatively inexpensive.
In practical terms, a "fuel" is any substance which can hold enough energy to power a reasonably-sized vehicle for a distance of at least several hundred miles while fitting into a container small enough to be carried by that same vehicle. The key fact here is not that the fuel provides the energy, but that it contains it. An extension cord plugged into a household power socket can provide a lot of energy, but you can't take it very far. Conversely, most batteries (anything from a miniscule Lithium watch battery to an automotive wet-cell battery) are quite portable, but cannot hold enough power to move a vehicle any useful distance.
The most important thing about oil, which is even more true once it is refined into chemicals such as gasoline (petrolium) and diesel, is that it has a high energy-to-volume ratio – in other words, you can pack a lot of energy into a relatively small space. There are very few substances which approach this density, and most of them create other problems -- hydrogen, for example, is far more combustible than gasoline, and is extremely hazardous to transport in quantities large enough to be useful as a fuel.
To the best of our knowledge, naturally-occurring oil is formed by the actions of heated chemicals under extreme pressure over "geological time scales" (millions of years), and therefore is not something of which we can quickly make more. Once the naturally-occurring supplies have been exhausted (), we will need to have found either an alternate method of production or a better means of storing energy.
(to be written) Some data here: 
- Fuel Oil Synthesis: If gasoline and diesel could be synthesized from renewable resources, all we'd have to deal with would be the environmental effects
- Alternative Portable Energy: There are other ways to store energy so that it can be used portably; the trick is getting the energy density high enough.
- 2006-02-17 Kids Build Soybean-Fueled Car that gets 50 MPG (2006-03-01 slashdot)
- BP Oil Statistical Review of World Energy 2005: a lot of data, available in multiple formats
- Friends Committee on National Legislation
- UK fuel protests: Wikipedia: signs of a flagging supply
- "Peak Oil" Wikipedia category: articles related to anticipated decline in oil supply
- Hubbert peak theory: Wikipedia: much discussion of the situation
- 2005 Energy Crisis: Wikipedia
- George Monbiot: "oil" articles
- 2006-05-10 Are You Ready for the Energy Crash? by Jan Frel, AlterNet: "The biggest obstacle to getting our petro-dependent society to change its wasteful ways is collective insanity."
- 2006-04-19 Tilting at Windmills (slashdot): popular opposition to alternative energy projects
- 2006-01-26 Ethanol can replace gasoline with big energy savings, comparable impact on greenhouse gases
- 2005-10-08 Seventy dollars a barrel? Relax, it'll come down. (2006-04-22: Oil breaks through record $75)
- 2005-10-07 House passes bill to boost refinery capacity: how does the Bush Administration handle the increasing fuel crunch? They lower our pollution standards to make sure we're running at top speed when the last few drops come out of the ground.
- 2005-09-02 U.S. drivers won't cut back on gas: or, more to the point, they can't, without substantial changes in the culture
- 2000-11-13 The Peak of Oil Production and the Road to the Olduvai Gorge by Richard C. Duncan, Ph.D.: somewhat iffy argument with some fairly specific data and conclusions, including expected crisis years. Appears to be the basis for the DieOff site's major premise.