This text introduces the basic equations of the theory of structures. Conventional presentations of these equations follow the ideas of elastic analysis, introduced nearly two hundred years ago. The book is written against the background of advances made in structural theory during the last fifty years, notably by the introduction of so-called plastic theory. The emphasis throughout is on the derivation and application of the structural equations, rather than on details of their solution (nowadays best done by computer), and the numerical examples are deliberately kept simple.