The earth's vast and varied cold environments could be rich sources of psychrophilic microorganisms growing at 5°C or below. Unfortunately, the diversity, physiology and potential of these organisms have largely been overlooked. This article focuses on psychrophiles and their cold-active enzymes and emphasizes how future studies could give basic insight into protein structure and could yield industrially useful enzymes. It presents an overview of the characterization of psychrophiles and their growth properties, a summary of biochemical work with coldactive enzymes, a description of comparisons of enzymes with different temperature optima, and a preview of uses for cold-active enzymes in biotechnology.