For a long time, studies of femtoscopic correlations have provided information about space-time characteristics of particle sources in high-energy collisions. Recently, the correlation functions have also been used to determine interaction parameters of correlated particles, which is especially important for short-lived particles, for which scattering experiments are impossible. The abundance of experimental data and their high accuracy require an improved theoretical approach to femtoscopic correlations. The aim of this paper is to systematically discuss relativistic effects and their role in femtoscopy. Since a general relativistic approach is currently unavailable due to serious theoretical difficulties, the correlation functions must be computed in the center-of-mass frame where the correlated particles are mostly nonrelativistic. This requires transforming the source function to this frame, the consequences of which are discussed. Since the deuteron formation has a similar physical origin to femtoscopic correlations, relativistic effects are also discussed in the former process. The considerations are illustrated with calculations of some correlation functions and the deuteron coalescence coefficient to demonstrate the magnitude of relativistic effects. It is also argued that a discrepancy between the coalescence coefficient computed with the source radii inferred from the baryon-baryon correlation functions and the experimental data can be removed by taking into account the relativistic elongation of the source radius in the center of mass of correlated particles.

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• Particle interactions
• Particle production
• Phenomenology