This paper investigates the exponential observer design problem for one-sided Lipschitz nonlinear systems. A unified framework for designing both full-order and reduced-order exponential state observers is proposed. The developed design approach requires neither scaling of the one-sided Lipschitz constant nor the additional quadratically inner-bounded condition. It is shown that the synthesis conditions established include some known existing results as special cases and can reduce the intrinsic conservatism. For design purposes, we also formulate the observer synthesis conditions in a tractable LMI form or a Riccati-type inequality with equality constraints. Simulation results on a numerical example are given to illustrate the advantages and effectiveness of the proposed design scheme. IMPROVED EXPONENTIAL NONLINEAR OBSERVER DESIGN 3959 to conservativeness [21][22][23][24][25][26][27][28]. Hu [21,22] first considered the observer design problem for onesided Lipschitz nonlinear systems. Following Hu's work, further results on the observer existence conditions and reduced-order observers can be found in [23] and [24], respectively. The stabilization problem for one-sided Lipschitz systems via state or output feedback was investigated in [25]. However, it should be emphasized that Hu [21] actually introduced a modified one-sided Lipschitz condition in which the nonlinearity is scaled via a fixed symmetric definite matrix P (see Definition 1, i.e., P -one-sided Lipschitz condition). As has been pointed out in a recent reference [26], this modification makes the design problem tractable but affects the value of the one-sided Lipschitz constant (i.e., v p strongly depends on P ) and introduces additional constraints on the Lyapunov function. Therefore, observer design for systems with nonlinearities satisfying P -one-sided Lipschitz constraints is still an open problem.More recently, Abbaszadeh and Marquez [27] introduced the standard one-sided Lipschitz condition (Definition 2) and reconsidered the observer design problem. To avoid scaling of the one-sided Lipschitz constant, they presented an alternative approach based on a second condition known as the quadratically inner-bounded property and have provided a solution using matrix inequalities [26]. Following this line, further improved results on the design of full-order and reduced-order, unknown input, and exponential observers can be found in [29][30][31]. The discrete-time one-sided Lipschitz observer was developed in [29,32]. However, although this approach does not require scaling of the one-sided Lipschitz constant, it clearly introduces an additional restriction into the design and its consequent conservativeness [26]. Indeed, the nonlinear plants considered in the aforementioned references [27][28][29][30][31][32] are only a subclass of one-sided Lipschitz nonlinear systems, which are also quadratically inner bounded (Figure 1).From the previous analysis and discussion, we know that most of the existing references studied the problem of observer design for...