Massive mechanical oscillators have recently been measured and controlled in the quantum regime, providing a testbed for investigating the limits of quantum mechanics and its possible interplay with gravity. The stabilized entanglement of massive mechanical oscillators has been measured both indirectly and directly. Further, sensing of the motion of a mechanical oscillator beyond conventional quantum limits has been demonstrated. There exist further proposals for the realization of enhanced force sensing and many-body quantum state control in optomechanics, and problems in optomechanics have spurred the development of novel theoretical techniques.