The behaviour of a clay element during the sampling operation is first analysed in the light of the limit state theory. The stress variation and the resulting distortion of the clay element during sampling is equivalent to a destructuration of the clay, and its mechanical properties are greatly affected. The causes of sampling disturbance are analysed and the principles which should govern the design of a good quality sampler are reviewed in the light of previous studies. It is shown how the reshaping of the sampling tubes can improve the sample quality. The new 200 mm sampler designed at Université Laval uses the overcoring technique to eliminate suction and to insure the recovery of large samples without a piston. The quality of these large samples has been proven to be equivalent to block samples. The advantages and limitations of the Laval sampler are discussed.