Insecticide resistance is an important aspect of pest control on both crop pests and vectors of animal diseases. Resistance can be caused by a number of mechanisms, one of which is enhanced detoxification of the insecticide by metabolic enzymes. Synergists may be used in insecticide formulations to inhibit metabolic defences in the insect, allowing the insecticide to reach its target site and kill the insect, thus enhancing the effect of the insecticide. This PhD project investigated the use of the synergist piperonyl butoxide (PBO) in combination with natural pyrethrins (tank mix) and as a pre-treatment prior to application of pyrethrins, as methods of enhancing the efficacy of the insecticide. The insects studied were Myzus persicae, Bemisia tabaci and Musca domestica. Results showed that the combination treatment (tank mix) was at least as good as, and sometimes better than, the pre-treatment. This is unlike the situation for synthetic pyrethroids where pre-treatments have been shown to be more effective than tank mixes. It is proposed that for natural pyrethrins, PBO aids the penetration of the pyrethrins into the insect, and this enhancement effect is greater than full inhibition of the metabolic enzymes. In some cases, the tank mix enabled less insecticide to be used to achieve 50% mortality in resistant insects, compared to a susceptible population treated with pyrethrins alone. A novel laboratory assay was developed to enable the screening of botanical extracts for their ability to inhibit esterase enzymes. This was used to test a range of compounds and those showing esterase inhibition were also screened for their ability to inhibit cytochrome P450 activity. The competency of some of these compounds as synergists was also tested in vivo with some showing potential activity both in vitro and in vivo.