Have you ever wondered how a cup of hot coffee cools down when you leave it on a table? Even though there is no external forcing, convection heat transfer still occurs due to the convection currents that exist within the ambient air owing to density gradients. This is referred to as natural convection and is the focus of this course. Natural convection plays an important role in many industrial applications such as power generation, HVAC systems and electronics cooling. Moreover, it is the driving force for many natural processes such as ocean and atmospheric currents. In this course, we will start with understanding the physics behind natural convection in greater detail and follow it up with a discussion of the boundary layer profiles in natural convection. We will then discuss how natural convection varies in external and internal flows by analyzing several canonical cases such as the flow over a plate, the flow between parallel plates, flows insides enclosures, etc.