If, as we have stated, our goal in plant disease management is to keep disease development below an acceptable level and therefore to understand disease progress in quantitative terms, it will be necessary to find some kind of mathematical model to describe the epidemic. Our model should show how some variable, preferably something that we can count or measure in the real world, changes with time. From the management perspective, keeping track of new infections perhaps would be the most useful, but actually observing these microscopic events in most cases would be too difficult to be practical. Monitoring inoculum is also very useful for disease management, and for many diseases practical techniques have been developed for quantifying inoculum. We will begin our discussion, therefore, with changes through time in the amount of inoculum available to cause disease.