Planning systems have been an active research topic within Artificial Intelligence for over two decades. There have been a number of techniques developed during that period which still form an essential part of many of today's planners. This paper introduces the techniques, attempts to classify some of the important research themes in Al planning and describes their historical development. There
... [Show full abstract] has been a recent surge of interest in planning systems at both the research and applications levels. This paper should act as a brief introduction to the large volume of literature on the subject. An extensive bibliography is provided and cross-referenced throughout the text.