A helicopter is a complex nonlinear system and also an under actuated system with fewer independent control actuators than degrees of freedom to be controlled, making the control difficult. There is a growing interest in the modeling and control of such systems using nonlinear dynamic models and nonlinear control. Analytical techniques based on Lyapunov theory are then used to design the controller and still the design can become extremely complex. Hence the existing control methods use linearization techniques on the actual nonlinear dynamics of the plant and linear control techniques. The resulting performance may not be satisfactory, especially when the system is subjected to unknown and sudden disturbances. In this thesis, we present a new Nonlinear and Adaptive controller design which uses the actual nonlinear model of the helicopter and not a linearized version. The design methodology basically involves making the combined dynamics of the helicopter and the controller resemble the dynamics of a nonlinear time varying electrical circuit having the required properties using a process similar to reverse engineering. The circuit template in turn is formed from well defined time varying and/or nonlinear electrical elements and using proper interconnections. The kind of elements used and the general form of the dynamics derived will depend upon the application. For example in the helicopter case, the closed loop dynamics of the helicopter and the controller expressed in terms of the error variable should point to a NLTV circuit with only passive elements. For this, the reactive elements should have their relaxation points (the points where the stored energy is zero) at and only at the origin. Also the stored energy should be monotonically increasing. We can bring in any knowledge including the structure that we have about the plant being controlled in enhancing the circuit.