The operating voltage is 3.3 V, +/-.3 V . . DO NOT EXCEED THIS VOLTAGE. many Arduino shields are dual voltage. Make sure that your shield will work at 3.3 V. Do not use any shield that is strictly 5 V. The shield voltage should come from the I/O reference pin from the Arduino board itself.
Repeat, Do not exceed this voltage for any I/O pin. you will damage your board.