Processing natural language such as English has always been one of the central research issues of artificial intelligence, both because of the key role language plays in human intelligence and because of the wealth of potential applications. Many of the knowledge representation and inference techniques that have been applied successfully in knowledge-based systems were originally developed for processing natural language, but the language-processing applications themselves have always seemed far from being realized. The special series on natural-language processing is an attempt to bring language processing and its applications into focus/spl minus/to demonstrate techniques that have recently been applied to real-world problems, to identify research ripe for practical exploitation, and to illustrate some promising combinations of natural-language processing with other emerging technologies. Each of the four articles in the series provides some insight into the state of the art and conveys the practical significance of recent research in the field. >