The contents of this book are based on lectures given at the Mee ting on Mathematical Aspects of Finite Element Methods, held in Rome, December 10-12 1975, at the "Consiglio Nazionale delle Ricerche" (C.N.R.) organized by the letituto per le kpplicazioni del Calaolo "Mauro Picone" and Laboratovto di Analisi NumerieaThe subject of this meeting is of particular interest owing to the importance that the Finite Element Method has in many fields of enginee ring, not only from the point of view of research but also in the indu strial routine. It is well known that this method has been developed by engineers as a concept of structural analysis.