Entrainment models are developed for laminar axisymmetric flow in viscous jets, plumes, and wakes, and an application is made to the ascent of laminar plumes in a stably stratified environment. The entrainment flux scale is obtained using order-of-magnitude arguments, and in each case is shown to be proportional to the kinematic viscosity ν with the exception of thermal plumes at Prandtl numbers less than unity when it is proportional to the thermometric conductivity κ.