In Waterfall model of the Software process, there is no chance to revisit the phases of SDLC once completed. Hence it is basically considered as a Fixed Approach. Since the requirements keep changing we as we don’t take the approval from the stakeholders, or development & implementation takes a long time, there is a lot of rework to be done. Here’s when the Agile process of Software development comes into the picture as Waterfall model of the software process is highly perilous & expensive and less efficient than Agile methodology.