<p> Let K = (K, v, ... ) be a model of a model-complete theory, T of valued fields. We characterise, for certain definable subsets S of K^n, the collections of S-T-integral definite and S-T-infinitesimal definite rational functions. Specifically, we consider subsets S defined by both integrality and infinitesimality conditions for the theories of algebraically closed valued fields, p-adically closed fields, two model-complete theories of valued D-fields and in two model-complete theories of henselian residually valued fields.</p> / Thesis / Doctor of Philosophy (PhD)