The key to understanding the catalytic action of an enzyme is the study of reaction velocities and not equilibria. Nevertheless, equilibrium and steady-state are two important states of any dynamic system. Both have much relevance to the understanding of enzyme mechanisms and hence metabolism. This chapter will elaborate on these concepts. There are many analogies/models to describe these states which include ponds and rivers. We will look at two simple setups to understand what is meant by equilibrium and steady state, before going into details.