When I'm carrying my backpack, inside a zippered pocket that's inside the main (also zippered) compartment. I'm quite confident no pickpocket can get to it without me noticing. A couple years ago I saw a pickpocket grab the wallet from an old man's back pocket and run away. Since then, if I need to keep it handy, I always keep it in a front pocket. I made it myself out of a tyvek envelope, so it's as thin as 3 or 4 cards.