<p>Tomato leaf miner, <i>Tuta absoluta</i> (Meyrick), is among the most destructive invasive pests of tomato worldwide, with chemical insecticides remaining central to its control despite widespread resistance development. Intensive and often uncoordinated insecticide use has driven resistance evolution across multiple modes of action, highlighting the urgent need for more consistent and reliable resistance monitoring approaches. In this review, we summarize the major bioassay methods used to assess insecticide resistance in <i>T. absoluta</i>, including primarily leaf-dip assays together with topical, residual film, and diet-incorporation approaches. We discuss their strengths, limitations, and suitability for resistance monitoring programs. The review also examines key approaches used in dose–response analysis, such as LC₅₀ and LC₉₀ estimation, resistance ratio calculations, and the application and evaluation of diagnostic concentrations (DCs) based on baseline susceptibility data. Findings from synergist bioassays and validated target-site mutations are also discussed to explain the biochemical and genetic basis underlying resistance phenotypes. Special attention is given to the practical use of diagnostic concentrations for large-scale resistance surveillance and decision-making in the field. We further discuss how DC-based monitoring can support insecticide resistance management (IRM) programs through insecticide rotation, reduction of selection pressure, and integration with biological control strategies. Finally, we highlight emerging tools, including molecular diagnostics, RNAi-based technologies, and regional monitoring networks, as promising approaches for improving resistance surveillance. Overall, this review synthesizes current knowledge on resistance monitoring and management strategies for <i>T. absoluta.</i></p>