In multi-period farm planning problems involving uncertainty, the usual case in the real world, planning must involve a continuous process of re-evaluation. In this dynamic situation it is necessary to know the number of periods to include in the decision model to ensure optimality. This is referred to as the planning horizon. In practice, planning models used are frequently simplifications of reality so the ability to determine the planning horizon under a range of situations is an important facet of planning. This paper contains a survey of methods of determining the horizon and offers comments on the approach necessary to resolve the cases for which methods have not been developed.