We are interested in using ultra-sensitive micro- and nano-mechanical resonators to probe quantum states. We study the quantum behavior of small mechanical structures, their coupling to single electron states, to spin states, to light, and to the larger environment around them. Sensors able to detect the tiny forces arising from single charges or spins allow the study of a wide class of problems in condensed matter physics. Measurement techniques with unprecedented sensitivity, including torque magnetometry and force-detected magnetic resonance, have emerged as important applications of nano-mechanical devices. Improved understanding of these phenomena may lead to new high resolution nano- and atomic-scale imaging techniques.