Case-based reasoning (CBR) is an approach to problem solving that emphasizes the role of prior experience during future problem solving (i.e., new problems are solved by reusing and if necessary adapting the solutions to similar problems that were solved in the past). It has enjoyed considerable success in a wide variety of problem solving tasks and domains. Following a brief overview of the traditional problem-solving cycle in CBR, we examine the cognitive science foundations of CBR and its relationship to analogical reasoning. We then review a representative selection of CBR research in the past few decades on aspects of retrieval, reuse, revision, and retention. R. LÓPEZ DE MÁNTARAS ET AL.

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Treatment of¯ax shive with sulfuric acid produced a carbonaceous material which has been used to remove mercury(II) from aqueous solution. The kinetics of sorption follows a ®rst order reaction equation with the rate of sorption being higher for the wet material than for that which had been previously dried. Sorption of mercury depends on the pH of the aqueous solution with maximum uptake occurring in the pH range 6±7. Sorption capacity also increases with the increase of temperature. The presence of other metal ions such as K , Na , Mg 2 and Ca 2 decreases Hg(II) uptake capacity. A high capacity which exceeds the cation exchange capacity was observed, cumulative Hg(II) sorption exceeding 1 gg À1 . This arises from the reduction of mercury(II) to mercury(I) chloride and elemental mercury from chloride media and to elemental mercury from nitrate media. This was con®rmed from the identi®cation of deposits on the carbon surface by scanning electron microscopy and X-ray diffraction. The reduction of mercury was accompanied by the oxidation of the carbon which was con®rmed by the evolution of carbon dioxide. This observation was also supported by changes in the infra-red spectrum of the carbon after reaction. The sorption mechanism is discussed.

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We briefly examine case-based planning starting with the seminal work of Hammond. Derivational analogy represents an important shift of technical emphasis that helped mature the techniques. The choice of abstraction level is equally important. We conclude by discussing theoretical underpinnings and by providing some pointers to current directions. CHEFBy developing the first case-based planner (CHEF), Hammond helped to define the case-based approach to problem solving and to explanation (Hammond, 1989(Hammond, , 1990. Given a set of goals and