Time and again, tourists come home from vacation having had their wallet stolen. In many countries, the locals have certain tricks to distract you so they can steal your wallet. Keeping it in your back pocket is potentially a bad idea, because you can’t see it or feel it if someone moves it.
Be smart while traveling in unfamiliar places: transfer your wallet to your front pocket. Consider attaching a clip to it to tie it to your belt loop. This should prevent any petty theft from happening, and you will always know where your wallet is.