The mock theta functions were first stated by S.~Ramanujan in his last letter to G.~H.~Hardy in 1920. In a nutshell, these are functions that mimic the behavior of theta functions in some sense, but have asymptotic behaviors that are unlike those of ordinary theta functions. In particular, they are not modular. In this talk, we will provide an overview of the historical development of this subject and discuss works by G.~N.~Watson, G.~Andrews and most recently, K.~Ono.