Yosr, Madi, and Debbabi (2014) specified in the taxonomy of Software defined networking about the role of controllers. We have heard about remote controllers, air conditioner controllers and various others. The problem is we have heard about them but never tried to figure out what is their purpose internally. Here we are not going to talk about Controllers in general but a specific kind of controller that is SDN Controller. Not to worry if the talk starts to get tech savvy, everything would become vivid as you go on to read the entire unit. Each and every question about this technology will be handled with utmost simplicity and ease as we move into the fathoms. In this chapter we will look at the following major topics: Downloaded 4/15/2022 9:44:49 AM Nova Southeastern University IP Address: 137.52.76.29 The Heart and Brain of SDN Controllers: Their functions Usage and benefits of Controllers as a single entity. Criteria to evaluate a Controller Types of Controllers. Benefits of different types of Controllers Comparing each controller with the previous and finding the optimal one for a particular scenario. Now, what does it comes to your mind when you read the word control? Probably to manipulate someone, or have an authority over someone. Similarly, a Controller Has an Authority Over the SDN Network A Controller is at the core of the SDN network. It acts as a middle man and connects the switches and the routers at the lower end to the applications or the business logic at the upper end. This helps in providing an abstraction to the application layer about the inner functioning of the system. Wibowo, Gregor, Ahmed and Chavez (2017) clearly state the importance of a controller over a network. Controller manages flow control to enable a kind of a network which manages situations on its own, somewhat intelligent by nature. There are numerous protocols on which a controller can be based on. One of the most popular is Open Flow. In effect, It somewhat plays a role of an Operating System. You must have used a computer 