By their very nature, compact heat exchangers allow an efficient use of material, volume, and energy in thermal systems. These benefits have driven heat exchanger design toward higher compactness, and the trend toward ultra-compact designs will continue. Highly compact surfaces can be manufactured using micro-machining and other modern technologies. In this paper, unresolved thermal-hydraulic issues related to ultra-compact designs are discussed, and the status of the technologies required for the production of ultra-compact structured surfaces is summarized. This review article includes 67 references.