Some basic principles of mechanics were reviewed, using examples to show how they are important to practical design. Equilibrium is the fundamental requirement of many engineering calculations, and it is important to ensure that all forces can be transmitted into the ground that is able to resist them; computer programs that consider only part of the equilibrium may be insufficient. Modern finite element programs are easy to use, but it is essential that users cling to a sound grasp of soil mechanics and behaviour, and constantly ask themselves if the model in use is suitable for the current task; in particular dilation is important in controlling the strength of undrained or confined soils. Engineering courses tend to concentrate on stress states and stress analysis, but strain is much easier to observe and sometimes gives important warnings of impending problems. Finally, the principle of superposition was carefully taught, with emphasis that it only applies in linear situations; but in practice, this limitation is often forgotten.