The security of currencies come from their underlying demand. Gold is used as a currency, first and foremost, because it has strong intrinsic value, and second because it has physical properties that make it convenient to divide and store. If gold had no independent demand it wouldn't be considered for a currency.<p>The US dollar has even more demand. All Americans are required by law to purchase it and own it for the purpose of paying their taxes. If they do not, the most powerful police and military will lock them in prison. This is the base upon which the rest of the system is built.<p>Bitcoin, to its good fortune, is also backed by independent demand: crime. Because of crime, and its adeptness at enforcing agreements, BitCoin has a strong tangible base upon which it can grow its ecosystem. The biggest threat to BitCoin is not a sudden loss of interest by the masses, but a competing currency that better serves crime and takes BitCoin's market share.