Abstract: Modular Vehicles (MVs) are an emerging type of intelligent transportation carrier that can dynamically adjust capacity and functions through coupling/decoupling of vehicle units or reconfiguration of standardized modules, thereby enhancing system efficiency and task adaptability in passenger transport, freight transport, and special operational scenarios. With the advancement of autonomous driving, intelligent transportation systems, and related enabling technologies, planning and operational optimization of MV-based transportation systems have become important research topics. This paper systematically reviews the research progress on MVs in transportation planning and operations. It first traces the evolution of the MV concept and summarizes major design paradigms and technical features. It then reviews advances in modeling, operational organization, and optimization-based decision-making across passenger transport, multimodal coordination, freight and mixed passenger-freight transport, and special scenarios such as military, rail, and medical applications. Building on these application contexts, the paper identifies key research issues, including dynamic scheduling and capacity matching, flexible route and feeder service design, passenger-freight collaboration, coordinated energy replenishment, and resource allocation in complex task environments, while synthesizing the main research approaches and methodological characteristics. Finally, based on a systematic summary and critical review of the existing literature, this paper establishes an overall research framework for MV planning and operational optimization, with the aim of providing references for future research and engineering practice in this field.