Autonomous mobile robots need the capability to reason from and about spatial knowledge. Due to limitations in the prior information and in the perceptual apparatus, this knowledge is inevitably affected by uncertainty. In this paper, we discuss some techniques employed in the field of autonomous robotics to represent and use uncertain spatial knowledge. We focus on techniques which use fuzzy sets to account for the different facets of uncertainty involved in spatial knowledge. These facets include the false measurements induced by bad observation conditions; the inherent noise in odometric position estimation; and the vagueness introduced by the use of linguistic descriptions. To make the discussion more concrete, we illustrate some of these techniques showing samples from our work on mobile robots.