This book contains the lecture notes prepared for the Second Advanced School on Artificial Intelligence (EAIA'90), held in the Hotel de Turismo, Guarda, Portugal, October 8-12,1990. The EAIAs are organized every two years by the Portuguese Artificial Intelligence Association. This year the focus was on Natural Language Processing. There are some divergences between the present texts and the actual lectures. Antonio Zampolli was most unfortunately unable to come because of health problems. Antonio Porto kindly accepted at very short notice to replace ZampoUi's lectures by presenting the last evelopments of his Intermediate Semantic Representation formalism, for which no text is included here. Luis Damas gave lectures on Constraint Logic Grammars, but did not produce a text for inclusion here.