Main features-DEREK allows: -specify a description of the system as a set of differential equations, initial conditions, as well as the parameters in equations -find errors in the description of the system, specifying precisely the place and nature of the error; -automatically choose the parameters of the numerical method for solving the system; -build visual graphics for solutions and any dependent on solutions variables. DEREK itself can scale to field plots for best display them, but admits to perform a layout manually -explore the three-dimensional image of the phase trajectory. DEREK can rotate a three-dimensional phase trajectory, as well as zoom in and out of her. DEREK also includes some special methods for the analysis of dynamical systems: -construction of a "family" of solutions that depend on one or two parameters, or on a set of different initial conditions; -building a "Poincare sections" - the set of points of intersection of the system trajectory with a given plane. -calculation of "Lyapunov exponents" - sets of numbers that characterize the behavior of the system at infinite evolution and do not depend on the initial conditions (except, perhaps, some exceptions).