This paper presents the theoretical formulation of generic frequency response functions (FRFs) for continuous systems associated with various forms of actuation and sensing methods. The generic actuator and sensor eigenfunctions are identified and interpreted physically. The FRF can then be expressed in a conventional modal format. The testing procedure by roving the sensor with the fixed actuator results in the sensor eigenfunction after the extraction of modal parameters. In contrast, roving the actuator with the fixed sensor results in the actuator eigenfunction. The physical meanings of the extracted mode shapes are shown to be characterised by the form of the actuators and sensors. The lateral vibration of a one-dimensional beam problem is illustrated to demonstrate the applications of various forms of actuators and sensors to structural modal testing. Three types of actuators and three types of sensors are considered, and the feasibility of structural testing for applying any combination of the actuators and sensors is shown. This work provides the theoretical base for applying various forms of actuators and sensors to structural modal testing.