The incidence, influencing factors and mechanisms of resistance to insecticides from a range of chemical groups were examined in UK and European populations of the glasshouse whitefly, Trialeurodes vaporariorum (Westwood). Toxicological assessments of populations from a range of plant production glasshouses and comparisons with the responses of a laboratory susceptible strain disclosed levels of resistance to pyrethroid, organophosphate, insect growth regulator (IGR) and neonicotinoid insecticides. Responses to conventional compounds indicated varying levels of resistance, potentially reflecting disparate usage between collection sites. All strains examined possessed resistance to the IGR, buprofezin; some populations were virtually immune to this commonly used control agent. Selection experiments demonstrated reciprocal crossresistance between buprofezin and a further IGR, teflubenzuron, both of which are frequently incorporated into integrated pest management (IPM) programmes for this species. Results for the leading neonicotinoid, imidacloprid, revealed resistance in both UK and European strains, representing the first documented cases of neonicotinoid resistance in this species worldwide, and the first in any insect species within the UK. The lethal effects of vapour emitted by applications of buprofezin and the anti-feedant effects of imidacloprid were demonstrated in T. vaporariorum for the first time. The potential consequences of these factors for both the control and selection of resistance were highlighted. Mechanistic studies using electrophoresis and kinetic spectrophotometer readings showed that neither non-specific esterases nor modified acetylcholinesterases were involved with resistance to either pyrethroid or specific organophosphate insecticides.