This comprises a large field - from rational engineering to evolutionary biotechnology, from the combination of natural protein domains in a modular fashion to the de novo design of new proteins. Over the last years, evolutionary biotechnology has seen a rapid development. Whether this is considered protein engineering in the proper sense is a matter of definition. Already a number of economically very successful applications of improved proteins exist - these range from thermostable proteases for laundry detergent formulations, to therapeutics, such as the cardiac infarction drug Reteplase. But the really spectacular successes are more obvious in basic research.
Proteins are complex systems.
We have known for more than thirty years that protein structure and function are encoded in the sequence. Since this time we have been trying in vain to predict a protein's structure from the sequence of its gene, or change the sequence in a rational way to obtain a new structure or function. This is the protein folding problem. Thus it isn't surprising that evolutionary engineering as a problem solving strategy has largely replaced rational approaches. From a perspective of biotechnology, this is perfectly justified.