Modal decomposition of optical fields as a concept has been in existence for many decades, yet despite its clear
applications to laser beam analysis it has nevertheless remained a seldom used tool. With the commercialization of
liquid crystal devices, digital holography as an enabling tool has become accessible to all, and with it modal
decomposition has come of age. Here we outline the basic principles of modal decomposition of laser beams with digital holograms, and review recent results on the modal decomposition of arbitrary optical fields. We show how to use the information to infer the intensity, phase, wavefront, Poynting vector and orbital angular momentum density of the light. In particular, we show how to achieve optimal modal decomposition even in the absence of key information about the field, such as its scale and wavefront. We demonstrate the techniques on optical fields from fibers, diode-pumped solidstate lasers, and structured light by laser beam shaping.