When, why and how to test spreadsheets
  Testing is a vital part of software development, and spreadsheets are like any other software in this respect. This paper discusses the testing of spreadsheets in the light of one practitioner's experience. It considers the concept of software testing and how it differs from reviewing, and describes when it might take place. Different types of testing are described, and some techniques for performing them presented. Some of the commonly encountered problems are discussed. 
Louise Pryor