Measurement plays a central role in quantum theory. This thesis explores fundamental and practical limits to the sensitivity of measurements of the mechanical motion of a macroscopic object. Optical measurements of mechanical motion are at the core of the research efforts in this thesis. Extremely sensitive optomechanical measurements can be performed in systems in which a mechanical vibrational mode is co-localized with an optical cavity field. In this work we perform measurements on sliced nanobeam photonic crystal cavities, that operate in the regime in which the mechanical frequency is smaller than the optical cavity line width and exhibit large photon-phonon coupling strength.