As primary degradation products of persistent plastic waste, microplastics (MPs, <5 mm) and nanoplastics (NPs, <1 μm) have emerged as a critical global environmental concern, with their ubiquitous distribution documented across aquatic, terrestrial, and atmospheric ecosystems. With annual plastic production exceeding 460 million metric tons, their widespread presence in environmental matrices and biota—from marine organisms to human tissues—poses significant, yet incompletely understood, threats to ecological integrity and public health. This paper systematically reviews the state-of-the-art detection techniques, environmental fate processes, and remediation strategies for MPs and NPs. In terms of detection, we cover microscopy, mass spectrometry, flow cytometry, chromatography, and spectroscopy, emphasizing hyphenated techniques (e.g., FT-IR microscopy, Raman spectroscopy) for enhancing sensitivity and specificity. Fate studies reveal that MPs/NPs exhibit long environmental persistence, undergo bioaccumulation and trophic transfer, and can act as carriers for organic pollutants and heavy metals. Removal techniques include physical (membrane filtration, adsorption), chemical (coagulation, advanced oxidation), and biological (biochar immobilization, microbial degradation) approaches, each with distinct advantages and limitations. This review synthesizes current knowledge gaps and provides a scientific framework for developing integrated management strategies to mitigate plastic pollution risks.