Agile Wheeled Mobile Robots for Service in Natural Environment INTRODUCTION Wheeled locomotion still prevails in the 21 st cen- tury because of high efficiency on various types of grounds, mechanical simplicity, and ease of control. In natural environment, though, wheels are challenged by other locomotion modes, such as tracks, that provide excellent grip on low cohesion grounds, or legs, that allow moving on irregular environment and cross obstacles. This chapter presents recent developments that improve the capacities of wheels with additional systems such as innovative suspensions that can have reconfigurable states or additional mobilities. Another interesting solution is to create hybrid systems between wheels and legs, by putting a wheel on a leg or even by transforming the rigid frame of the vehicle into a mechanism. These new paradigms are particularly inter- esting in a time where electric actuators can be · · Section 2 analyzes the skid-steering pro- cess at low speed of a 6x6 wheeled vehicle (Fauroux, Charlat, & Limenitakis, 2004a, 2004b; Fauroux, Vaslin, & Douarre, 2007; Fauroux & Vaslin, 2010). Skid-steering has similarities with what happens on tracked vehicles: because of transverse friction, a lot of energy is dissipated during steering. This section proposes a model of the vehicle behaviour as well as experi- mental results, with the general goal to both understand the phenomena and also improve these category of very robust vehicles. Section 3 deals with another low speed challenge: obstacle crossing. In this part, OpenWheel i3R, an agile modular mo- bile robot designed by the authors with articulated chassis and four motorized wheels (Fauroux, Chapelle, & Bouzgarrou,