An amazing amount of misinformation on this thread. I'm answering for the US. I don't know about other countries.
Of course you have to pay taxes on your profits in cryptocurrencies. This is well established.
Whether you have to pay taxes on exchanges of one cryptocurrency for another depends on whether such transactions can be claimed as "like kind" transactions. Whether "like kind" applies to exchanges between cryptocurrencies is controversial. I think that the IRS will eventually rule that these transactions are _not_ like kind and I've read extensively on the issue. Others disagree.
If you choose to treat exchanges between cryptocurrencies as like kind transactions you have to declare them as such on IRS form 8824! Absolutely everyone who trades cryptocurrencies ignores this (except me, of course).
If the IRS rules that these transactions are not like kind, and you have filed form 8824, you are likely to be OK except that you will owe taxes, interest, and possibly penalties. If you haven't filed form 8824 I wouldn't want to be you.
Not a lawyer - just my well researched opinion.
JUST FOR US:-
After doing lot of research this is the only correct answer in this thread unless of coarse you want to do tax evasion. I have talked to several lawyers/accountants and they are all of the opinion you cannot apply in kind for Virtual Currency ( though IRS has not clarified anything). If you do apply in kind you have to file 8824 as sp_skeptic mentioned but soon IRS might rule on it. You will pay tax one way or another even if you defer your tax.. Also remember if you have had assets > 10000 on any international exchange you need to file FBAR as well.
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. We will go to moon when Ian Balina and Sasha Ivanow will be promoting Encryptotel. 
Way to go Enric and team 
If 1 faircoin == 100Euro then I will retire haha 
