Preface. Part I: Classical Methods. 1. Approximation Theorems for Valuations of Fields. 2. Valuations on Commutative Rings. 3. Ordered Groups and Homomorphisms. 4. Approximation Theorems for Multistructures. Part II: Categorical Methods. 5. Categorical Logic. 6. Approximation Theorems in Categories. Bibliography. Index of Notation. Index.