Abstract: An improved compressor that is designed to minimize shock and vibration during start-up includes a plurality of pistons, each of which is positioned for movement within a cylinder in order to compress a gas, and conventional motive structure for driving the pistons. Advantageously, the compressor includes a sensor for sensing, from a condition that exists within the compressor, when the compressor is in a start-up phase. The sensor is designed to be operative regardless of the position of the pistons during start-up. Pressure relief structure, responsive to said sensor, is provided for relieving pressure in at least one of the cylinders when the sensor indicates that the compressor is in the start-up phase. This minimizes shock and vibration during the start-up.