<p>This paper presents a comprehensive review of the dynamic influence of secondary components on rotor systems, which play a critical role in determining system stability, vibration characteristics, and overall performance. While primary rotor elements such as shafts and discs have been extensively studied, the effects of secondary components—including bearings, seals, couplings, and support structures—have not been systematically synthesized in existing literature. This review critically examines the individual and coupled effects of these components on rotor-dynamic behaviour, including stiffness variation, damping characteristics, and induced nonlinearities. Emphasis is placed on understanding how interactions among secondary elements influence critical speeds, stability thresholds, and vibration response. The study consolidates findings from recent theoretical, numerical, and experimental investigations to provide a unified perspective on rotor-system behaviour. Key research gaps are identified, particularly in the modelling of coupled multicomponent interactions and the integration of advanced materials and smart systems. The insights presented in this work aim to support the development of more accurate predictive models and improved design strategies for high-performance rotor systems in modern engineering applications.</p>