An integrated pest management (IPM) program is a systems approach that requires an understanding of the overall agroecosystem. The interaction between different pest categories (weeds, insects, pathogens, nematodes), as well as their management practices, should be examined, understood, and taken into consideration in the design of IPM systems. Several studies have shown that plant pathogens are affected by other pests and their management practices. Herbicide application has often been cited as an example of a management practice that affects plant pathogens and disease development in various cropping systems. The activity of herbicides can extend beyond their target organisms and inhibit spore germination or mycelial growth, alter the level of phytoalexins, or interfere with other physiological processes in plants. This paper summarizes the published reports on direct effects of herbicides on plant disease and provides insights for future research on this aspect. Examples are drawn from some common agricultural herbicides and adjuvants. The discussion on various findings on herbicide and crop diseases re-emphasizes the fact that pest management in agriculture requires a systems approach. Although it is difficult to come to a common consensus on the effect of these chemicals on crop diseases or pathogens, this paper provides an overview of several interactions between herbicides and crop diseases and pathogens in various cropping systems. Knowledge of such interactions can help in the design of IPM systems.