Enzymes are biocatalysts that mediate many reactions necessary for life. They are remarkable because they perform their functions at ambient temperature and pressure in a highly substrate-selective fashion in the presence of scores of structurally related compounds. Gene sequence information, along with an increasing number of protein structures, reveals that many enzymes arose from a subset of common ancestors. This underscores the high degree of functional plasticity exhibited by individual enzyme folds and suggests that existing enzymes can be further adapted to perform desired biotransformations. The poor performance of some naturally occurring genes in transgenic settings, along with theoretical considerations suggesting newly evolved enzymes are likely to have poor kinetic properties and stability, provides a rationale for engineering enzymes to perform specific reactions in planta. The techniques of enzyme engineering represent a powerful new addition to the arsenal of the metabolic engineer. Over the last decade, enzymes have been tailored to perform specific transformations or to become adapted to perform efficiently under specific conditions. There are as yet few examples of the effects of such technologies being applied to plants. However, because plants represent the primary route of terrestrial fixed carbon, the potential impacts of enzyme engineering, and ultimately metabolic engineering, are far reaching. Using these techniques, plant scientists will be able to create rationally engineered crops that will suffer decreased losses from insects and disease which will accumulate desired forms of reduced carbon to meet the increasing and changing needs of society.