Crop disease epidemics are the result of the complex interaction between the pathogen, the host, and its environment. The best way to control disease in crops is to eliminate the disease by growing resistant cultivars. The physical environment affects all stages in the life cycle of plant pathogens and much of the effort in plant pathological research is devoted to the study of these changes. Temperature and moisture are the two most important microclimatic factors in the direct interaction of host and pathogen, although radiation can have also have an influence, while wind and rain play dominant roles in dispersal processes. Leaf wetness, temperature, wind, and to a lesser extent radiation, are the most important microclimatic variables which influence the development of epidemics of most crop pathogens. The chapter considers each of these variables and how they may be measured or estimated for use in disease control systems.