The paper is devoted to the analysis of seismic input characteristics from the point of view of their importance for engineers. It is noted that the design input does not need to have an external resemblance to the real one but it must provide for some properties of real actions. The paper considers three groups of seismic input characteristics: kinematic, spectral and energy ones. The stability of the characteristics under consideration is analyzed within the seismic intensity on the MSK scale. Among all characteristics, peak acceleration, peak velocity, harmonicity coefficient, Arias intensity, and absolute cumulative velocity are highlighted. It is noted that the kinematic characteristics significantly depend on the prevailing accelerogram period, and many energy characteristics are stable within the seismic intensity under consideration and can describe it. It is noted that for calculating structures under the design earthquake action, the kinematic characteristics should be fundamental for the engineer, and for calculating structures under the destructive (maximum design) earthquake action, the energy characteristics are fundamental. Two new seismic input characteristics are introduced, which are based on the response spectrum of the work of plastic deformation forces and on the structure damage spectrum.