The piezoelectric and piezoresistive effects of InAs nanowires are experimentally demonstrated for the first time and are observed to strongly depend on the NW crystal structure. While single-crystalline < 0001 > oriented wurtzite nanowires exhibit remarkable piezoelectric and piezoresistive effects, they are negligible in single-crystalline wurtzite < 11 (2) over bar0 >, zinc blende < 011 >, < 103 >, <(2) over bar(1) over bar1 > oriented nanowires, and significantly suppressed by the presence of stacking faults.