The similarity parameter is connected to basic symmetry properties of the collisionless Vlasov equation and is thus the relativistic plasma analog of the Reynolds number in fluid mechanics. Gordienko showed that in the relativistic limit ( a 0 ≫ ≫ 1 ) the laser-plasma dynamics depends on three dimensionless parameters: ω ω 0 τ τ , R ω ω 0 / c and S , where τ τ is the duration of the laser pulse and R is the typical radius of the laser waist. The main result of the relativistic similarity theory can be summarized as follows: if the parameters of the interaction (plasma density and laser amplitude) change simultaneously so that the S parameter remains constant, the dynamics of the electrons remains the same.