Abstract : A robust and reliable attitude estimator is a key technology enabler for the development of autonomous aerial vehicles. This paper is an introduction to attitude estimation for aerial robotic systems. First, attitude definition and parameterizations are recalled and discussed. Then, several attitude estimation techniques – ranging from algebraic vector observation-based attitude determination algorithms to dynamic attitude filtering and estimation methodologies – are presented and commented upon in relation to practical implementation issues. Particular attention is devoted to the applications of a well-known nonlinear attitude observer (called explicit complementary filter in the literature) to aerial robotics, using a low-cost and light-weight inertial measurement unit, which can be complemented with a GPS or airspeed sensors.