Quasi-equilibrium Tropical Circulation Model (QTCM) [Neelin and Zeng 2000] is an atmospheric model of intermediate complexity. It is based on the quasi-equilibrium framework, which has the effect of constraining the vertical temperature profile in tropical convecting regions. Due to these constraints, part of the model solution can be approximated by projecting the solution onto a single vertical mode. This makes the model computationally economical, and also helps in analysis of the model and its behaviour. In this talk, I would give a brief introduction to QTCM and its equations, while talking about the more general quasi-equilibrium concepts and how they are used in QTCM. I will also talk about how simple models can be developed from QTCM.