Recently, cross-layer design has received significant interest in the communications research community. In cross-layer design, functions in several layers of a reference model are optimized simultaneously with respect to a critical dimensioning system resource, e.g. network capacity or battery life-time. Properly used, cross-layer design can give significant performance gains, while incorrectly used; it can lead to an unintentional reduction in modularity. We treat different aspects of cross-layer design and its impact on design of efficient communication systems. We report ongoing research efforts in cross-layer design, and discuss different definitions and interpretations of cross-layer design. We propose a definition of cross-layer design as follows: Cross-layer design is protocol design by deviation from a particular reference model, through extra information flow or in other ways, with the purpose to optimize the performance with respect to a specified critical resource for a well delimited system. A new reference model that considers radio communication properties and the need for flexible radio signal processing while providing sufficient structure to support modularity is needed for wireless communications. Finally, we give examples of areas where cross-layer design is of interest for military communications and point out new possibilities for tactical radio networks and sensor networks.