Ground-penetrating radars (GPRs) are one technology being investigated for the detection of buried landmines. Full experimental simulations for potential GPRs are complicated, time consuming, and expensive. An alternative is to perform simulations using computational electromagnetic techniques, such as the finite-difference time-domain method (FDTD). Until recently, such simulations were not possible because of the limitations on computer memory and speed. Results are presented from two FDTD simulations for GPRs designed for mine detection. One of the GPRs uses continuous-wave signals (the 'separated aperture' or 'waveguide below cutoff' detector for nonmetallic mines), and the other uses baseband pulses (a proposed detector for small antipersonnel mines). These simulations are fully three-dimensional and include all of the details of the detector (antennas, reflectors, feeds, etc.), lossy earth, and mine. Results from the simulations are compared with experimental measurements made with model detectors.