Creating a true revolution in the financial field, Cryptocurrencies have exploded in popularity in recent years. Nonexistent before the 2008 crisis, they have been popular since 2009, as a consequence of a loss of investor confidence in traditional financial systems. Used only by a handful of insider programmers when they originated, crypto-currencies now attract individuals, private investors, venture capital funds, and, since the beginning of this year, specialized hedge funds. While Bitcoin was one of the only cryptocurrencies in circulation in 2009, there currently are several hundred. The major ones are Bitcoin ($107 billion), Ethereum ($29 billion), Ripple ($7.8 billion) and Litecoin ($3 million). The current capitalization of all cryptocurrencies is in the order of $165 billion, which is equivalent to the size of the PepsiCo or Walt Disney corporations. This would rank them 29th on the S&P 500. The growing interest in these alternative currencies has pushed the value of some to unexpected heights. They are widely referenced in the media, but often misunderstood. What are cryptocurrencies, and why do the authorities see them as significant threats to the financial ecosystem?