**Introduction of models in software engineering -:** A model in software engineering come’s in many shapes, sizes and styles.This is very important to understand that a model is not the real world but It is a human construct that help us to understand real world systems. All models has an information input, an information processor and an output of expected results.

**Types of sdlc model-:** For understanding the Software Development Process **SDLC **provide some models in software engineering. These are the

**(1)- Waterfall m**odel -: This is the oldest sdlc model. waterfall model is very easy to understand. In the waterfall model has many phase. In this model all the phase are step by step completed. Means if the first stage completed then the next steps comes. It don’t go to return previous steps.

**(2)-Spiral Model in software engineering -: **Spiral model is another one of the most important **SDLC model **that provides Risk Handling support to sdlc model.Like any model’s its represent diagrammatic.It looks like a spiral with many loops. The exact number of loops of the spiral is unknown and different from project.

Every loop of the spiral is called a Phase of the sdlc process.The exact phase of a spiral model are depending on the project risk and its depend project manager dynamically determines the number of phases so the project manager has an important role to develop a product in the spiral model.

**(3) V shaped model-: ** This is know us the verification and validation model.The V-Model is a similar as the waterfall model and it is based on the association of a testing phase for each corresponding development stage. In other words we can say that the v shape model every phase is a development cycle this is a testing phase.

**(4)Agile Model in sdlc-:** It allow a company to develop software in small segments.Means the agile model was developed in the groups.The agile model was design for changing in quickly.The main purpose of this model was to facilitate quick project completed.As a result of this model are release small changes to users more frequently. Every release phase as a foundation for the next one. This methodology differs from traditional software development, where a company must complete each phase before moving on to the next one

**(5)Iterative sdlc model -:** This model does not need full list of requirement before when we are starting the project. The development team process to starting the development with the requirements to the functional part which can be describe later. The process is regular repeating and allowing to make new versions of the product for every cycle.

Each iteration includes the development of a separate component of the system and later then this component is added to the functional developed in early phase .If we talk in math terminology the iterative model is a realization of the sequential approximation method that means a gradual closeness to the planned final product shape.