Sliding controllers have recently been shown to feature excellent robustness and performance properties for specific classes of nonlinear tracking problems. This paper examines the potential use of sliding surfaces for observer design. A particular observer structure including switching terms is introduced for both systems in companion form and systems in general form. The analysis of systems in companion form shows that sliding observers have promising properties in the presence of modeling errors and sensor noise, as highlighted through two numerical examples. Extensions to general nonlinear observable systems are also discussed.