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?"  String theory ideas 
(new scalar particles and extra dimensions) and other notions hint that 
Newton's Inverse-Square Law could break down at distances less than 1 mm. 
Furthermore, the observed dark energy density corresponds to a length 
scale of 85 microns that may have fundamental significance. I will review 
the motivations for testing the Inverse-Square Law, and discuss recent 
experiments with torsion balances, small oscillators and micro-cantilevers. 
Our torsion-balance experiments at separations down to 60 micrometers 
exclude gravitational-strength Yukawa interactions with length scales 
greater than about 57 micrometers (approximately the diameter of a human hair),
and set a robust 95% confidence upper limit of 46 micrometers on the size 
of an extra dimension. Problems encountered in probing these short length 
scales will be discussed.