There is an astonishing simplicity in some processes of intense laser-matter interactions. At high field amplitudes, the atomic electrons acquire energy from the driving laser, which can be released in the form of high frequency coherent radiation, in a process known as high-order harmonic generation (HHG). The lack of conventional lasers at high frequencies drives the technological interest of HHG as a basis for coherent short wavelength sources. Until very recently, intense laser technology was limited to near-infrared wavelengths (around 800 nm), consequently, the HHG conversion was limited to the extreme ultraviolet. Nowadays, thanks to the development of mid-infrared lasers, the current limit falls in the soft X-rays region. In addition, the HHG spectrum is emitted in the form of ultrashort pulses, with typical durations in the attosecond timescale, that are considered the shortest bursts of coherent radiation ever created. The aim of this work is to make an original contribution to this field, that consists on developing theoretical methods that can be directly compared with the HHG experiments. We pay special attention to the propagation of the higher-order harmonics