Publisher Summary This chapter provides a holistic view of array systems that perform the radar, electronic wadrfare (EW), radiometric, and communications functions. As all array systems share the same theory, this fundamental premise seems eminently reasonable. There are many similarities between array systems used by different communities of researchers, the differences between them are also considered and emphasized. The chapter also demonstrates that all phased arrays share the same theory and that there is a continuum between radar imaging arrays having short integration times and radiometric/radioastronomy arrays having short or long integration times. While the instantaneous beam derived from a densely populated radar array can have low sidelobes and the corresponding beam of a sparsely populated radioastronomy array is almost useless because it has high sidelobes, the latter sidelobes can be lowered to an acceptable level in quasi-real time. Similarly, the gain of a thinned array is very much lower than that of a filled aperture having the same extent. The linkages between arrays performing a variety of functions not only enrich and deepen one's understanding of array systems but also allow a designer imbued in one discipline to utilize the techniques and approaches developed by another discipline. Lastly, the defense of high-value platforms or the efficient operation of airports depends on array systems that derive interdependent and interrelated data.