Seperation of concerns (SoC)   is a key software design principle that mainly states;  - different concerns of your design should not be colocated,  - meaning that different concerns should be clearly isolated with boundaries. Then what is a concern ? A concern is a way of grouping of items in a specific software development scope. For example; - In the context of object-oriented programming languages such as Java, the concerns are seperated into objects. - When the context is of service-oriented design, then the concerns are seperated into services. - In the architectural design patterns such as MVC, we seperate concerns into a kind of grouping: model, view and controller. - The question of why we put the css styles into seperate files is another good example of seperating concerns of styling from the content. - In a modular software program, different modules are seperated concerns in a way. The idea of seperating responsilities in Single Responsibility Principle (SRP)