Instructional systems design (ISD) involves a systematic process for the assessment and development of training solutions, designed specifically for the purpose of formal training delivery. There are two widely recognized instructional design models in use today by both educational institutions and corporate training functions. The most traditional is the ADDIE model, of which there are several variations. The second is the agile model, whereby there are several variations, including rapid application development, rapid content development and the successive approximation model.