Torsion balance experiments: A low-energy frontier of particle physics

We review recent mechanical experiments that test some of the most basic principles of physics including the weak and strong forms of the Equivalence Principle, the gravitational inverse-square law, and Lorentz invariance. The very high sensitivity of these tests allows one to place interesting constraints on string-theory inspired conjectures about new Yukawa forces from the exchange of very light scalar, pseudoscalar or vector particles, large extra dimensions, the chameleon mechanism, non-commutative spacetime geometry, and Planck-scale Lorentz violation.