I. Introduction  
II. An Overview of Radio Systems  
A. Active Radio Systems  
B. Passive Radio Systems  
III. Symbiosis and Symbiotic Radio  
A. Biological Symbiosis  
B. Radio Symbiosis  
C. SRad Enabling Symbiotic Relationships  
IV. Symbiotic Communication  
A. Overview  
B. Symbiotic Relationships  
VI. Applications  
A. Human Centric  
B. Machine Centric  
VII. Open Problems and Future Directions  
A. Transmitter Design  
B. Receiver Design  
C. Network Design and Management  
D. Channel Modelling and Estimation  
F. Performance Analysis and Metrics  
VIII. Conclusion