This dissertation analyzes economic issues related to the use of agricultural pesticides and antibiotics. The efficacy of both these chemical compounds depends on the existence of susceptible targets. However, through natural selection, their utilization increases the frequency of the genes resistant to the pesticide or drug in the target population, and it decreases the available biological capital of genetic susceptibility, increasing resistance;The aim of this dissertation is to analyze the characteristics of optimal resistance management strategies. An empirical analysis is offered in case of bioengineered Bt corn, which underlines the importance of pest mobility, and the externalities it causes, in the development of resistance;Chapter II consists of a general theoretical model on resistance development that incorporates a spatial dimension. This allows the analysis of the impact of pest mobility on the effectiveness of refuges, areas in which the pesticides are not used to preserve the existence of susceptible pests. A discussion of eradication policies and of the role of cross-resistance is included;Chapter III presents an empirical analysis of the role of pest mobility in the development of resistance in the case of Bt corn. The model includes the existence of mandatory (structured) refuges for farmers planting Bt crops and analyzes the impact of incomplete market penetration on resistance development and profits. If part of the crop production area is seeded with non-bioengineered seed, these fields act as an unstructured refuge. The current policy is effectively based on a 100% market penetration. This is not necessarily a realistic assumption. We explicitly evaluate the role of market penetration on resistance;Chapter IV concentrates on the optimal use of existing antibiotics and on the optimal time path of investment in the development of new technologies. The intertemporal allocation of susceptibility and the development of alternative technologies are important social issues, particularly in the case of antibiotics, because health can be considered a necessary good. The analysis focuses on the role of endogenous technological and the investment of resources in alternative technologies;Lastly, some general conclusions on the importance of externalities and technological change in the mining of susceptibility are presented.