The legal status of cryptographic currencies (in particular, of bitcoin, the first decentralized cryptocurrency) varies substantially from country to country, and is still undefined or changing in many of them. While some countries have explicitly allowed their use and trade, others have banned or severely restricted it. Some countries that allow or tolerate cryptocurrencies have chosen to treat them as foreign currencies, sometimes with status of legal tender; while others have denied their currency status but consider them as a valuable property. Reasons that have been alleged for restricting or banning cryptocurrencies include their use for illegal trade and other illegal payments, protection of the national currency, and protection of the nation's financial system. Many central banks, even in countries that officially allow their use, have issued warnings to the public about their risks.

http://en.wikipedia.org/wiki/Legality_of_Bitcoin_by_country