Targeted illumination manifested by source, spectra, intensity, brightness and regimen became major factors in modern poultry management. Light spectra affect growth in meat type birds both in ovo and post hatch. Broilers and turkeys in ovo photostimulated with green light (GL) gained significantly more weight than birds incubated under dark conditions. This chapter defines the cellular and molecular mechanism behind this effect. In ovo photostimulation have a stimulatory effect on the proliferation and differentiation of satellite cells and a uniformity of muscle fibers, by direct effect on embryo's somatotropic axis activity, resulting muscle growth. In a recent study, we found that in ovo GL photostimulation can be done during hatching period only with same growth acceleration results. The chapter suggests that embryos are highly sensitive to GL photostimulation a phenomenon that can be a tool for acceleration of muscle growth in meat type birds.