This paper studies the guided waves in honeycomb sandwich structures and explores the ability of guided waves for the debonding damage detection. Both the finite element (FE) simulations and laser vibrometry experiments are used. A three-dimensional (3D) FE model is built to simulate the guided waves in a honeycomb sandwich plate. The simulation results show the guided waves in the structure depend on the wave frequency. At low frequencies, the global guided waves propagate in the entire sandwich, while leaky guided waves dominate in the skin panel at high frequencies. To further understand the guided wave propagation fundamentals, laser vibrometry experiments are performed. The waveforms, time-space wavefields, and frequency-wavenumber spectra obtained from the experiments are used to unveil the wave propagation features. The experimental results confirm the leaky guided waves. Moreover, the experimental results show the complex wave interactions induced by the honeycomb core. When the debonding between the skin and honeycomb core presents, the guided wave amplitude increases, and the wave interaction with the honeycomb core reduces.