One of the best tax-avoidance tactics in the late Roman Empire was to sell yourself into slavery. You didn't really have to work as somebody's slave, of course — it was more like rock star Hotblack Desiato being "dead for a year for tax reasons" in Douglas Adams' wondrous confection "The Hitch-Hiker's Guide to the Galaxy." But with the legal status of slave, you were exempt from taxation.
Nowadays the legal manipulations used to avoid taxation are less dramatic, but they are spectacularly effective.
James Henry, former chief economist at business consultancy McKinsey and a member of the board of directors of Tax Justice Network, has just published a report, "The Price of Offshore Revisited," that estimates the amount of wealth hidden in tax havens by the super-rich at a minimum of $21 trillion. It might be as much as $32 trillion, he adds, but greater precision is impossible when the whole point of holding money overseas is to keep it secret.