The main objective of this master thesis was to experimentally investigate the boundary layer under a solitary wave. Experiments of solitary waves on constant depths were conducted, varying the amplitude in order to investigate if and when the experimental results start to deviate from the theoretical ones. Another interest was to presumably observe instabilities and the growth of Tollmien-Schlichting waves in the boundary layer under the solitary waves. The experimental work included measurements of the surface elevation and the velocity field at the bottom by means of an ultrasonic probe and optical method of flow visualisation, respectively. The surface elevation data provided by the probe was later on evaluated by means of MATLAB in order to ensure that the solitary waves were generated with very high reproducibility and in agreement with theory. The post-processing of the pictures was done in DigiFlow and MATLAB, and both Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) were applied for comparison purposes. The surface elevations and velocity profiles by both PIV and PTV are in very good agreement with theory. The considered boundary layers under the solitary waves turned out to be stable. Consequently, it naturally proved difficult to further investigate any Tollmien-Schlichting waves.