The transmission-line-matrix method is a numerical technique for solving Maxwell's equations in the presence of complex environments. A general-purpose simulation program, based on the three-dimensional symmetrical-condensed TLM model, is presented. Previous applications of the simulation program include the analysis of half-space and remote-sensing problems, and calculation of the radar cross section of finite-sized conducting materials and objects. The simulation program is applied to the characterization of aperture-coupled microstrip structures. Numerical results, pertaining to the analysis of aperture-coupled microstrip lines, offset-aperture-coupled microstrip lines, and aperture-coupled microstrip-patch antennas, indicate the accuracy and utility of the program for analyzing aperture-coupled microstrip configurations. A discussion of the accuracy and efficiency of the approach is included, with limitations and possible improvements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>