Money laundering is the process by which dirty money is made clean. Let's first think about how money is made dirty in the first place. There are many ways to make money dirty. Think of practically any criminal or illegitimate activity. The money earned from those activities is dirty money. The biggest sources of dirty money, according to the UN office on drugs and crime are counterfeiting, illicit drugs, environmental crimes, human trafficking and weapons trafficking. Collectively, the crimes and illegitimate activities that make money dirty are known as predicate offenses. They are the crimes that precede the money laundering. So how big is the problem? Or put another way, how much cash is the criminal economy thought to generate each year? Nobody really knows, but global financial integrity estimated it to be in the range of 1.6 to $2.2 trillion per year. In 2017. That would mean the criminal economy accounts for between 2% and 5% of global economic output. So what form do the proceeds of crime usually take? Maybe not surprisingly, but the answer is cash. Dirty money is most commonly cash, and that creates a problem for the bad guys. What to do with all the dirty cash? They can't easily deposit it at a bank. Since banks are obliged to ask questions about the provenance of large amounts of cash, and to be on the lookout for suspicious transactions, the purpose of money laundering is to make dirty money indistinguishable from legitimately earned money and to make it bankable.