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Dynamical mechanisms to generate an ultralight axion of mass $\sim10^{-21}-10^{-22}$ eV in supergravity and strings are discussed. An ultralightparticle of this mass provides a candidate for dark matter that may play a rolefor cosmology at scales $10\, {\rm kpc}$ or less. An effective operatorapproach for the axion mass provides a general framework for models ofultralight axions, and in one case recovers the scale $10^{-21}-10^{-22}$ eV asthe electroweak scale times the square of the hierarchy with an $O(1)$ Wilsoncoefficient. We discuss several classes of models realizing this frameworkwhere an ultralight axion of the necessary size can be generated. In one classof supersymmetric models an ultralight axion is generated by instanton likeeffects. In the second class higher dimensional operators involving couplingsof Higgs, standard model singlets, and axion fields naturally lead to anultralight axion. Further, for the class of models considered the hierarchybetween the ultralight scale and the weak scale is maintained. We also discussthe generation of an ultralight scale within string based models. Here it isshown that in the single modulus KKLT moduli stabilization scheme an ultralightaxion would require an ultra-low weak scale. However, within the Large VolumeScenario, the desired hierarchy between the axion scale and the weak scale isachieved. A general analysis of couplings of Higgs fields to instantons withinthe string framework is discussed and it is shown that the condition necessaryfor achieving such couplings is the existence of vector-like zero modes of theinstanton. Some of the phenomenological aspects of these models are alsodiscussed.

An Ultralight Axion in Supersymmetry and Strings and Cosmology at Small Scales