This book attempts to (i) systematically review the state of the art pertaining to the knowledge on the various levels of structure and catalytic function of Iipases, and (ii) discuss in a simultaneously pragmatic and integrated manner the design and modification of Iipases and the technological aspects of the utilization of Iipases for desired practical purposes. In preliminary assessments of whether the current knowledge of Jipases (and users thereof) would deserve (and benefit from) comprehensive academic and scholastic efforts eventually leading to the production of a formal book, four major issues were addressed: -Need. Although several applications of Iipases have been developped throughout the latest two decades, innovative uses of these enzymes for a wide variety of organic syntheses and modification of existing fats and oils have increased exponentially during the latest five years. Reasons which may partly account for this trend are the increasing availability of Jipases from (genetically engineered) microbial sources coupled with their special capacity to act as catalysts at hydrophilic / hydrophobic interfaces. As a result of the structural characterisation effected during the same period, applications of Jipases can now be developed in a much more rational fashion; however, this purpose will not be fully achieved unless careful attempts to comprehensively clarify and integrate such advances are developed. -Importance. Once viewed as solutions in search of problems, Iipases have conquered a special position in the field of biotechnology since the advent of enzyme­ catalyzed reactions in nonaqueous (or micro aqueous) media.