Is it illegal to write on money?

Answer

In some countries yes - as you are defacing the currency and won't be accepted. In the USA it is, but the laws are rarely enforced.  

Answer

In Canada, it is illegal to hored money except in a bank, you can't keep it out of circulation. You cannot deface money because it is a legal document. No one really cares. In Canada and the states there is a fun game www.whereswilly.com and in the states theres a game called www.wheresgeorge.com. You can enter the serial number and track a bill.
Answer
Taken from The "wherehaveIbeen" website: This is, by far, the most commonly asked question we get and the answer is...

Not unless you're making them "unfit for recirculation" and, according to Title 18, Section 333 of the United States Code, this is defined as "mutilating, cutting, disfiguring, perforating, uniting or cementing together, or doing anything else to a bank bill with the intent to render such item(s) unfit to be reissued". So, if you're only writing the "www.WhereHaveIBeen.com" website address in neat lettering along one edge of the bill, you're certainly not making it "unfit for recirculation"!


"Of course, that's the American side of things. What about in Canada?"

At this time, there is nothing mentioned in either the Canadian Bank Act or the Canadian Criminal Code (see Part XII; Offences Relating to Currency) concerning the defacement of bank notes.

While the Bank of Canada (who decides whether a bill is to be removed from circulation and replaced) does not condone writing on bills, those bills remain legal tender and can be used in commercial transactions. For more information, please feel free to contact any member of the Bank Note Communication and Compliance Team of the Bank of Canada at 1.888.513.8212.