Cryptocurrencies have soared in popularity since 2008, with more than 1, 000 in existence today and an aggregate value greater than the market capitalization of IBM. But we are highly doubtful whether they will ever become mainstream currencies. The need for companies and individuals to pay tax receipts in government-issued currency, and the potentially unlimited crypto-money supply, pose significant barriers to widespread adoption. We think the sharp rise in crypto-currency valuations in recent months is a speculative bubble.