What is separation of concerns?

Separation of concerns means dividing software so different responsibilities are handled by appropriate components.

For example, an ASP.NET Core controller should primarily handle HTTP concerns and delegate business operations to application or domain services. Database queries should not be mixed throughout UI code, and email delivery should not be embedded inside unrelated business classes.

Good separation reduces coupling, improves testing, and makes changes more localized. It should be balanced against unnecessary fragmentation into tiny classes that add no meaningful boundary.