The identification of the complex molecular mechanisms that govern the development of acaricide resistance in mite pests allows for more efficient pest management strategies. In chapter 2 we present the first genome-wide sequence data set for the spider mite Panonychus ulmi, and investigate the molecular mechanism underlying spirodiclofen resistance. In chapter 3, we shed more light on the contribution of target-site mutations to resistance to Mitochondrial Electron Transport Inhibitors I (METI-Is) in the spider mite pest Tetranychus urticae. We gather a body of experimental evidence that supports a causal link between the H92R mutation and METI-I resistance. In chapter 4, we introduce a marker-assisted backcrossing approach to create T. urticae lines that share a common pesticide-susceptible genomic background, excluding 9 loci that carry mutations that are associated with resistance to avermectins, pyrethroids, mite growth inhibitors, and mitochondrial complex III inhibitors. By this approach, this study functionally validates previously reported target-site resistance mutations in T. urticae. Subsequently, in chapter 5, we further take advantage of these near-isogenic lines to investigate possible fitness costs that are associated with the well-characterized acaricide target- site resistance mutations. Findings reported in this thesis are important considering the global spread of acaricide resistance, and within an Integrated Pest Management context