This critical review analyzes methodologies used to collect, quantify, and characterize microplastics in both wastewater and drinking water. Researchers used different techniques at all stages, from collection to characterization, for quantifying microplastics in urban water systems. This represents a barrier to comprehensively assess the current loads of microplastic and to compare the results obtained by such assessments. The compiled studies address microplastic contamination using four types of sample collection techniques, four methods for processing samples, and four techniques for characterizing microplastics. This results in significant discrepancies in each of the following: (1) reported concentrations in both wastewater effluents and drinking water, (2) microplastic characteristics (i.e., size, color, shape, and composition), and (3) quality control and assurance procedures. Finally, this review qualitatively evaluated the reports by the completeness of their data based on a ranking system using five criteria: sample collection, sample processing, quality control, identification technique, and results. The results of this ranking system clarify disparities between the studies.