In this talk we present recent advances in the microscopic modelling of complex fluids such as polymer melts and solutions or wormlike micellar systems. In order to test thermodynamically admissible reptation models for dense polymeric systems, we outline, e.g., a microscopic foundation to its `primitive path' level description and discuss the validity of Fokker-Planck descriptions based on experimental data as well as numerical results obtained through nonequilibrium molecular and Brownian dynamics computer simulation.