Starting from the Mononobe-Okabe analysis, the seismic behaviour of gravity retaining walls is investigated. The importance of including wall inertia effects is demonstrated. The sensitivity of the results to changes in various parameters is explored: care must be taken in some ranges. For a moderately severe earthquake, it is shown that most walls will move, but that the movement is finite, and calculable. An approximate expression is given for the expected displacement. From this, a design approach is developed in which the designer chooses an allowable displacement, uses it to compute a design acceleration coefficient, and then computes the wall mass required.