This paper presents the basic relationships used in compiling a decision support system for wheat growers in the subtropical, prime-hard regions of Australia. The major factors addressed by this decision aid are climate variability, soil type and water status; N and P soil status and fertilizer addition; variety phenology, planting time and frost risk; weed infestation. The major decisions involved include fertilizer choice and quantity, choice of the variety development pattern to use for a given planting opportunity, and wild oat control measures. It demonstrates how the output from relatively complex, dynamic wheat models can be used at the farm level by introducing a number of factors (nitrogen status, frost risk, soil water status at planting, grain yield and protein records) measurable and specific to a given farm and/or paddock. The importance of such local measurements, and the consequent tailoring of the output to the user's conditions, on the user's confidence in, and ownership of, the final decision is demonstrated.