It is remarkable that small-scale experiments can address important
open issues in fundamental science such as: "why is gravity so weak?"
and "why is the cosmological constant so small?" I will  review recent
torsion-balance work that probes string-inspired ideas (large extra
dimensions, new scalar particles and chameleons, non-commutative
geometries, etc.). In particular I will discuss Equivalence Principle
tests with luminous and dark matter, short-distance tests of the
gravitational inverse-square law, and Planck-scale probes of Lorentz
symmetry, focusing on the experimental challenges and our strategies
for overcoming them. The precision attained in these experiments
provides laboratory proof that gravity is the dominant long-range
interaction between dark and luminous matter, demonstrates that any
extra dimension must have a size smaller than 44 micrometers, excludes natural values for the chameleon mechanism, and probes non-commutative
geometries up to a scale of 10^13 GeV.