This chapter emphasizes the value of dynamic weed seed germination on a daily basis, though other types of useful models have been designed with different purposes. Simulation of weed seed behavior as a function of the thermal environment is complicated because seeds present multiple responses to temperature that are often difficult to interpret. In some summer species, rising temperatures during late spring and summer induce secondary dormancy, which is evinced by a progressive increase in minimum temperature for germination. The rate and extent of the seasonal changes in dormancy depend on the thermal conditions to which the seeds are exposed. Totterdell and Roberts hypothesized that the loss of dormancy during winter of the seeds of Rumex obtusifolius and R. crispus is the result of two subprocesses: relief of primary dormancy and induction of secondary dormancy. It considers the influence of temperature on exit from dormancy and germination when water supply is not limiting.