Many different forms of the phasic conservation equations of two-phase flow have been presented in the literature. Unfortunately, there is still a lot of controversy as to 'best' form for two-fluid modeling. In addition, while there have been many attempts to constitute the intraphase and interfacial closure laws to achieve closure, ill-posed models are still the rule rather than the exception. The purpose of the paper is to show how various popular forms of the two-fluid model are related, and to summarize the current state-of-the art in the modeling of interfacial and intraphase closure laws. Moreover, new closure laws are proposed which attempt to account for the effect of dispersed phase (i.e., inter-partical) interactions. It is hoped that this paper will help stimulate multiphase flow researchers to redouble this effort to achieve accurate, properly closed, two-fluid models which can used for the mechanistic predictions of multidimensional two-phase flows.