Uncertainty is a universal and important aspect of geotechnical engineering and it is comprised of several different aspects. The most significant is likely that derived from spatial variability, where the properties vary from one location to another as a result of the processes that form the ground. Secondly, statistical uncertainty is a critical element of geotechnical engineering. Other important sources of uncertainty are those associated with the testing process itself, the transformation of the test results to design values, and uncertainties derived from human error. The paper discusses each of these uncertainties in some detail and provides examples and guidance on how to quantify and account for these in the geotechnical design process. The paper also presents two examples that demonstrate the power of statistical simulation in geotechnical engineering practice.