High-sensitivity searches for ``fifth forces'' can test a variety of models of new physics that produce new weakly coupled or short range (<< 1 mm) interactions. I will describe the development of a new class of force sensors based on optically levitated dielectric microspheres in high vacuum, which allow the detection of sub-attonewton forces acting on micron sized objects. Results from the initial application of these force sensors to search for millicharged dark matter particles bound in matter, and for interactions arising in certain screened scalar dark energy models, will be presented. I will also discuss the expected sensitivity of these techniques to search for deviations from Newton's and Coulomb's law at micron length scales, which can probe a variety of models of physics beyond the Standard Model.
