Informally, q-series can be considered as a summand containing (a,q)_n = (1-a)(1-aq)...(1-aq^{n-1}). Specifically, theta function is considered to be q-series and plays an important role in the theory of q-series. In this talk, I will introduce the history of the theory of q-series and prove some important identities involving theta functions. Finally, we will see how the approach from q-series are interacting with the mock theta functions. Speaker(s): Yiwang Chen (UM)