Plasma wakefield accelerators produce accelerating fields thousands of times higher than radio-frequency accelerators, offering compactness and ultrafast bunches to extend the frontiers of high energy physics and to enable laboratory-scale radiation sources. Plasma based wakefield accelerators have seen tremendous progress in the last years, now capable of producing few-femtoseconds high quality electron beams in the GeV energy range. In this presentation, I am going to review the fundamental idea of plasma based wakefield accelerators and highlight different charged particle bunch injection scenarios in these acceleration structures.