This paper compares four observers of vehicle sideslip angle. The first is linear and uses a linear vehicle model. Next observers use an extended nonlinear model. The three nonlinear observers are: extended Luenberger observer, extended Kalman filter and sliding mode observer. Modelling and model simplification are described, and an observability analysis is performed for the entire vehicle trajectory. The paper also deals with three different sets of sensors. Comparison is first done by simulation, and then observers are used on experimental data.