Fly-by interactions of galaxies are hidden drivers of galaxy evolution: The impulsive encounters are by far more frequent than and thus as important as direct mergers, yet hard to identify observationally. Here we present the key characteristics of fly-bys that are examined theoretically via cosmological N-body simulations. In particular, we use the simulations generated by a particle-mesh tree code, GOTPM, and investigate the statistics of galactic fly-by interactions, which are defined by the total energy of two halos of interest being positive and their minimum distances escaping mergers. We discuss (1) the rate of fly-by interactions (the Fly-by Rate, $R_f$) as functions of(a) redshifts, (b) halo masses and mass ratios, and (c) environments, and (2) their impact on galaxy evolution in terms of morphology and star-formation rate, in comparison to that of direct mergers.