Ans Waterfall Model: 1.The waterfall model is a sequential software development process, in which progress is seen as flowing steadily downwards (like a waterfall) through the phases of Conception, Initiation, analysis, design (validation), Construction, testing and maintenance. 2. The first formal description of the waterfall model is given by Winston W. Royce in 1950, however, popular in 1970, and further refined by Barry Boehm. 3. To follow the waterfall model, one proceeds from one phase to the next in a purely sequential manner. For example, one first completes requirements specifications, which are set in stone. When the requirements are fully completed, one proceeds to design. 4. Process structured as a cascade of phases where output of one is input of next. 5. Many variants of model depending on organization and specific project. However underlying phases are same for all. Why Waterfall model is advantageous than Adhoc Methods Ad-hoc Process Models-“Process capability is unpredictable because the software process is constantly changed or modified as the work progresses. Schedules, budgets, functionality, and product quality are generally inconsistent. Performance depends on the capabilities of individuals and varies with their innate skills, knowledge, and motivations. There are few stable software processes in evidence, and performance can be predicted only by individual rather than organizational capability.” So to overcome this problem waterfall model provides the following advantages: a. Waterfall model is simple to follow, however real projects rarely follows this approach. b. Iteration is not required. c. It is widely use because it is easy.