Lecture is devoted to an introduction to Digital Holography (DH). Three main DH methods are described: Phase Shifting DH (PSDH), Fourier Transform DH (FTDH) and Local Least Square DH (LLS). Advantages and disadvantages are shown and discussed. Application examples based on their advantages are shown. PSDH is a convenient digital holographic method introduced in [1]. Here φ(x,y) - object wavefront phase, Ip - holograms with shifted reference beam. Such approach does not require any complex calculating procedures and consider each pixel separately, resolution of this method is limited only by resolution of the registration system. PSDH disadvantage is that for elimination of the zero order and twin images at least two holograms have to be recorded. Therefore, this configuration cannot be used for wavefront reconstruction of fast going processes.   FTDH is a well-known reconstruction method of digital holograms with Fourier Transformation in the off-axis configuration. In this method Fourier Transformation applied for spatial filtering of the hologram in the frequency space to restore the object wave field. Thus, it is possible to filter the zero-order diffraction term and the twin image [2]. Filtering operation leaves Fourier transform of the image term in frequency space with no change and brings all the others to zero. After inverse FT the reconstructed wavefront is obtained. Resolution of the reconstructed wavefront is caused by the size of the filtered area, the bigger area size the higher resolution. But this size is limited by distance between image and zero orders, therefore resolution of the reconstructed wavefront always will be smaller than resolution of the registration system. Another widespread off-axis reconstruction method is LLS, where wavefront reconstruction is made in the registration plane, according to the conception of spatial phase steps [3]. This method requires the knowledge of a reference wave phase. If a flat wavefront used as a reference, only the knowledge of the angle between object and reference wavefronts is required. The key idea of this algorithm is that the hologram intensity and phase of the reference wave in the registration plane are changing a lot faster with spatial coordinates than unknown quantities: the object complex wave and amplitude of the reference wave. Therefore, the object wave and reference wave amplitude can be considered as constants inside the processing window. To determine them a system of M nonlinear equations must be solved, M is the number of considered points in the processing window. It is necessary for the successful application that the magnitude M is selected from the requirement that the neighboring pixels used in solving the system of equations must fall to one fringe at least. That is, it is assumed that the processing window covers at least one interference fringe.