In the last decade we have had a renaissance in molecular drag pumping, led by the design of hybrid turbopumps that exhaust at pressures above 10 Torr, to oil-free forepumps. The exhaust stages, which operate in laminar flow, may be of the Gaede, Holweck, or Siegbahn type. While Gaede provided the underlying model in 1913, upon which all are based, the Gaede design was not developed as a commercial pump until it was revived by Varian SpA, in Turin, in 1992. The neglect of the Gaede design was accompanied by 80 years neglect of the molecular drag model, in deficiencies which Gaede himself identified in his original papers. Pumps operate in continuum flow, which is a new field in vacuum science, extending the classical field of molecular flow. The appropriate mathematical tools have been developed in the fields of fluid mechanics (CFD), aerodynamics (RGD) and molecular simulation (DSMC). It is a challenge to extract from these fields a subset of theory that is appropriate and useful to molecular pumping. Many papers miss the fact that molecular drag pumps operate in the classic analytic regime of Couette-Poiseuille flow, and results need to be compared with the characteristics of this type of flow. Even G.A. Bird, author of the famous DSMC method, recommends that numerical simulations be guided by analytic models. Useful theory has been developed in the design of gas (journal) bearings, and floating magnetic recording heads. For molecular pumping, the inertial term in the viscous equations should not be neglected. Some papers calculate solutions with a form of slip-flow that has no physical correspondence, since in practice the molecules have full accomodation to the surface. This is an emerging field which has many opportunities for student research.