2.1 Introduction2.2 State‐Space Model2.3 The Concept of Observability2.4 Observability of Linear Time‐Invariant Systems2.5 Observability of Linear Time‐Varying Systems2.6 Observability of Nonlinear Systems2.7 Observability of Stochastic Systems2.8 Degree of Observability2.9 Invertibility2.10 Concluding Remarks