The elastic characterization of wood with acoustic methods is based on the assumption that its properties can be represenred by an equivalent homogeneous anisotropic continuum. It is generally accepted that solid wood is a material having orthotropic symmetry. Its elastic behaviour can be expressed by nine stiffness coefficients, or by nine compliances or by nine engineering constants such as : three Young's moduli, three shear moduli and Six Poisson's ratios. The stiffnesses can be measured with ultrasonic velocity methods, the compliances can be measured with static methods or can be deduced from the stiffnesses . The engineering constants can be deduced from stiffnesses or can be measured with acoustic resonance methods or with classic static methods. Either ultrasonic immersion technique or the direct transmission technique can be used for wood characterization. Immersion technique is more appropriate for laboratory testing while direct transmission technique is convenient for both laboratory and field measurements. The principal advantage of ultrasonic technique is the flexibility in measuring velocity and attenuation of ultrasonic waves. It is appropriate to observe that ultrasonic parameters are influenced by the requirements of sample preparation, coupling the transducers to the sample and the signal processing. The samples could be : trees, small clear specimens of solid wood or of wood composites, plates, etc. Some physical properties of wood can influence the parameters of propagating ultrasionic waves, such as : the specific anatomic structure at microscopic level, the annual ring structure, the moisture content, the density, etc. Depending on the nature of the investigation, all models acknowledge the precise determination of the elastic constants of the elements of the system, and the local characterization of the material. The recent development of acoustic microscopy could provide a very refined tool for nondestructive mechanical characterization of the structural elements of wood at the microscopic and submicroscopic scales.