The small carrier wavelength at millimeter-wave (mm-Wave) frequencies enables the implementation/nof a large number of co-located antennas. This paper exploits the potential of large antenna arrays at these/nfrequencies to develop a low-complexity directional modulation technique: Antenna Subset Modulation/n(ASM) for point-to-point secure wireless communication. The main idea in ASM is to communicate/ninformation by modulating the far-field radiation pattern at the symbol rate by driving only a subset/nof antennas in the array. Two techniques for implementing antenna subset selection are proposed. The/nfirst technique is simple where the antenna subset to be used is selected at random for every symbol/ntransmission. While randomly switching antenna subsets does not affect the symbol modulation for a/ndesired receiver along the main lobe direction, it effectively randomizes the amplitude and phase of the/nreceived symbol for an eavesdropper along a sidelobe. Using a simplified statistical model for random/nantenna subset selection, an expression for the average uncoded symbol error rate (SER) is derived/nas a function of observation angle for linear arrays. To overcome the problem of large peak sidelobe/nlevel in random antenna subset switching, an optimized antenna subset selection procedure based on/nsimulated annealing is then discussed. Finally, numerical results comparing the SER performance and/nsecrecy capacity of the proposed techniques against conventional array transmission are presented. While/nboth methods produce a narrower information beam-width in the desired direction, the optimized antenna/nsubset selection technique is shown to offer better security and array performance.