The success of acoustic non-destructive methods for quality assessment of wood products is related primarily to the understanding of the phenomena of ultrasonic wave propagation in the testing material and ultimately to defining how to use the results of the basic research in order to improve the technology. Acoustic techniques for defect detection in wood products (logs, lumber, veneer, wood-based composites) can be divided into two groups: the scattering-based techniques, which use wave travel time, frequency, amplitude ratio, waveform shape, etc. Most of these methods are performed using waves in the frequency range between 50 kHz and 2 MHz and imaging tomographic techniques, which seek to provide a picture of the discontinuity in the structure of the products. Ultrasonic tomography is performed with both velocity and attenuation as contrast-producing parameters.