This work is the overview of the existed approaches to design interval observer for different classes of systems with concentrated parameters. Principal differences of such devices from the conventional observers and the main purpose of the use in some systems with unknown variable parameters are considered. Several methods of design interval state observer, which depends on conditions of the considered system or model – time delays, parametric or signal uncertainties, discrete time systems, some nonlinearities or algebraic constraints, are analyzed. The efficiency of these methods are shown through computer simulation.