Significance: Turbine-based combined cycle (TBCC) engine is an ideal propulsion system for hypersonic flight, with a wide-speed range, large flight envelope, and horizontal takeoff and landing capability. The TBCC engine, comprising an air-breathing gas turbine and a ramjet, has become a key aspect of current and future aerospace research. When the TBCC engine operates across a wide-speed range (Ma 0-7.0), it undergoes a mode transition between the gas turbine and the ramjet. This transition requires coordinated operation among various components and subsystems, involving a broad disciplinary scope, high technical complexity, and significant implementation challenges. Consequently, the mode transition has become a critical bottleneck in the development of TBCC engines. Progress: This study systematically reviews the development progress of TBCC engines across various countries and analyzes the "thrust gap" phenomenon and the multi-component matching challenges that occur during mode transition. The review encompasses four key aspects: (1) Intake system design and regulation technology: Current mature approaches, such as boundary layer bleeding and vortex generators, offer limited adjustability, making precise and rapid flow control challenging. Axisymmetric intakes, favored for their simplicity in series-configured TBCC engines during mode transitions, still require enhanced variable-geometry capabilities to improve performance. Additionally, two-dimensional and three-dimensional inward-turning intakes provide greater regulation flexibility and effectively suppress inlet coupling interference; however, their control strategies within intake systems demand further in-depth investigation. (2) High-performance turbine and ramjet engine design, as well as rocket-assisted boost technology: Modified high-speed turbine engines utilizing inlet pre-cooling show greater potential, compared to newly developed ones, though their advancement hinges on the creation of lightweight pre-coolers that can operate across wide temperature ranges. For wide-speed ramjet technologies, methods such as rotating detonation combustion, advanced inlet designs, and combustion optimization can effectively extend the operational Mach number range. However, integrating these technologies into combined-cycle engines requires further in-depth research. While rocket-assisted thrust augmentation directly addresses the "thrust gap, " incorporating an additional rocket engine may introduce significant structural complexity. (3) Exhaust system design and regulation technology: Future directions focus on efficient aerodynamic profile design and active control of shockwave-boundary layer interactions. Regarding nozzle configurations, both two-dimensional and three-dimensional nozzles can satisfy the exhaust expansion requirements of combined-cycle engines. Two-dimensional nozzles offer simpler structures but pose significant challenges for aerodynamic integration. In contrast, three-dimensional nozzles provide superior performance and better integration potential with the overall propulsion system; however, they involve greater design, manufacturing, and control complexities. (4) The combined-cycle engine system integration, mode transition control, and experimental testing technologies: The United States has conducted relatively comprehensive research, having completed integrated engine model-level mode transition tests and comparative analyses of various control algorithms. Nevertheless, most existing studies remain theoretical or limited to model validation. Conclusions and Prospects: Many conducted mode transition experiments have not fully addressed the variable-geometry adjustment of the inlet and exhaust systems or the dynamic cooperative control of the fully integrated engine. Consequently, future research should prioritize cross-system integrated cooperative control for combined-cycle engines, the development of advanced test facilities capable of simulating wide-range flight environments, and full-scale engine validation of mode transition processes. Key future research directions include optimizing off-design performance and multi-physics coupling in intake system design, advancing rotation detonation combustion technology, developing three-dimensional nozzle control and multi-duct collaborative matching techniques, and establishing a full-chain research and development system for TBCC engines.