  The cell-model and its treatment on the Poisson-Boltzmann level are two
important concepts in the theoretical description of charged macromolecules. In
this brief contribution to the Proceedings of the NATO-ASI on ``Electrostatic
Effects in Soft Matter and Biophysics'', which took place in Les Houches from
Oct. 1-13, 2000, we provide an introduction to both ideas and summarize a few
important results which can be obtained from them. We outline the sequence of
approximations which ultimately lead to the cell-model. Then we present two
exact results, namely, an expression for the osmotic pressure and a formula for
the ion density at the surface of the macromolecule, known as the contact value
theorem. The next section provides a derivation of the Poisson-Boltzmann
equation from a variational principle and the assumption of a product state.
Afterwards we apply Poisson-Boltzmann theory to the cell-model of linear
polyelectrolytes. In particular, the behavior of the exact solution in the
limit of zero density is compared to the concept of Manning condensation.
Finally, we demonstrate how a system described by a cell model can be coupled
to a salt reservoir, i.e., how the so-called Donnan equilibrium is established.
