The future of aircraft lies in highly intelligent vehicles which are able to adapt themselves to different missions and take-off and land anywhere. Tailsitters, aircraft capable of controlled transitions between horizontal flight (like a fixed-wing aircraft) and vertical flight (like a rotary-wing aircraft), are one such form of vehicle. The focus of this research stems from a collaboration between the Surrey Space Centre and QinetiQ involving a new class of VTOL tailsitter - the QinetiQ Eye-On™ UAV - which offers uniqueness from all other known tailsitters in its use of twin helicopter rotors, elevons and a rudderless flying-wing design. A core objective of this research is to develop a control and navigation architecture capa.ble of handling the transitional flight regime in tailsitters and to understand the mechanism by which a transition can be controlled. Very little past research has addressed either of these in detail, often including only cursory modelling and simulation, no regard to how transitions can be 'shaped', and with no consideration of optimisation of transitions or whether their control laws are even robust. Another shortcoming of past literature is the minimal amount of experimental work undertaken which mostly uses only generic, simple single-propeller aircraft and does not consider transitional flight. Furthermore, examination of real-world applications where a tailsitter can be used has also been barely considered in literature. The limitations are addressed in three research divisions: I, II, III. {I} Development of a novel six degree of freedom (6-DOF) non-linear model with custom-designed numerical fluid dynamics, has allowed. high-fidelity simulation of all flight regimes to be performed. The developed control and navigation architecture is the first all-encompassing control architecture for the class of twin rotor tailsitters; it uses rudimentary low-level controllers and is capable of performing three different transitions: vertical to horizontal (V to H) , H to V (altitude elevation) and H to V (altitude invariant). The last of these is a ground-breaking discovery; transitions from H to V can be performed with virtually no increase in altitude. One improvement from past literature is that transitioning is undertaken in a closed-loop manner by commanding the vehicle to follow velocity and pitch setpoints. A carefully selected set of parameters has been devised to allow transitions to be shaped by transition time period, flight speed, sample size and smoothness of the control setpoint command. For the first time optimisation is applied to obtain ideal parameters for the transitions and robustness simulations stochastically consider environmental disturbances and variation of vehicle mass. [II] A comprehensive experimental framework has been developed tha.t uses various advanced testbed configurations to validate the control architecture, requiring the fusion of both aeroplane and helicopter technologies. Initially, an indoor motion capture testbed uses a series of precursory vehicles (including both quadrotor and Chinook) to pioneer taiisitter technology. A thrust testbed was also developed to explore thrust curve relationships and obtain optimal thrust zones for differing flight regimes. The outdoor testbed required the development of a complete self-contained autopilot system, including telemetry and ground station, which was tested in a progressive fl ight campaign spanning four flight locations across the UK. The experimentation forms the first demonstration of &-DOF untethered flight for the class of twin rotor tailsitters in VTOL, manual transitional and semi-autonomous transitiona.l modes. [III] In terms of systems analysis, two distinct civil and military scenarios are evaluated: linear asset monitoring, and perch and stare (which includes an innovative miniaturisation analysis) . The first thorough and realistic consideration is also given to the use of a reusable tailsitter v.'ith a docking station for staged exploration in extra-terrestrial environments. Industrially, the research programme extensively develops the technology necessary for autonomous flight of the UAV and extends from Technology Readiness Level (TRL) 2 to 6. Academically, significant contributions have been made to the field including: tailsitter modelling, transitioning methodology, control architecture, optimisation, testbed design, flight experimentation, systems design.