This book discusses the classical foundations of field theory, using the language of variational methods and covariance. It explores the limits of what can be achieved with purely classical notions, and shows how these classical notions have a deep and important connection with the second quantized field theory, which follows on from the Schwinger Action Principle. Its pragmatic view of field theory focuses on issues which are usually omitted from quantum field theory texts and catalogs results which are often hard to find in the literature.