The formulation of pesticides is a critical issue in ensuring improved efficacy, more effective targeting, and for achieving significant reductions in environmental impact. This chapter is concerned principally with efficacy and considers the wider issues of restricting entry of active ingredient and co-formulants into non-target organisms or the environment. It focuses on the effect of formulation factors on the uptake and translocation of pesticides in plants, drawing on examples from a range of active ingredient and adjuvant types, but with an overall emphasis of describing the general principles involved and the strengths and weaknesses of the current understanding of the mechanisms. The key processes of uptake and translocation have often been viewed from the narrow focus on the pesticide delivery system rather than in the context of the overall physiological behavior of the target organism. Pesticide formulations containing multiple components, including two or more active ingredients, may utilize quite different transcuticular pathways.