Analyzing dispersion in viscous media poses problems that differ significantly from those in low-resistance liquids. Their high resistance to movement disrupts widely used approaches including laser diffraction and DLS, which require particles to be sufficiently mobile for precise measurement. With rising viscosity, particle motion decelerates, causing extended measurement durations, diminished clarity, and false aggregates that distort outcomes. These obstacles demand tailored methodologies. A practical strategy is to dilute the sample using a chemically matched solvent, lowering resistance without altering particle integrity. Careless diluent choice may trigger unwanted chemical responses, particle swelling, or complete dissolution. If dilution is not feasible, high-shear pre-treatment can be applied to break up agglomerates and promote uniform dispersion prior to analysis. Reproducibility depends on consistent shear application, as too much force can fracture or deform fragile particles. A powerful method involves coupling rheological control with live optical analysis. These systems allow real-time monitoring of particle behavior under controlled shear rates, providing data not only on size but also on how particles respond to flow conditions. https://www.particle-raptor.jp/ is particularly useful for non-Newtonian fluids where viscosity changes with applied stress. Techniques like digital holographic imaging or flow-controlled automated microscopy provide direct particle imaging, bypassing the need for thinning. Coupled with specialized microfluidic platforms built for high-resistance fluids, these tools generate reliable data even in gel-like suspensions. Calibration against certified benchmarks is mandatory to maintain measurement fidelity. Temperature effects cannot be ignored, since viscosity and particle behavior often shift dramatically with thermal changes. Environmental temperature must be regulated, and data should be corrected for any thermal-induced viscosity or volume shifts. Data interpretation must consider the potential for polydispersity and non-spherical particle shapes, which are common in industrial slurries and polymer-based formulations. Advanced algorithms that analyze size and shape simultaneously improve the reliability of results. To ensure statistical validity, measurements should be acquired from various depths and regions due to the risk of layering in quiescent viscous systems. For industries including pharma, edible products, and high-performance ceramics, particle size accuracy governs efficacy, homogeneity, and manufacturing success. Therefore, selecting the right method depends not only on the fluid’s rheological properties but also on the nature of the particles and the intended end-use. Integrating multiple methods, verified by mutual agreement and iterative testing, delivers superior reliability. True success hinges on comprehensive knowledge of the material’s response and the operational boundaries of the instruments employed.