  In the first part of these lecture notes, new high-resolution observations of
small-scale magnetic flux concentrations are presented and compared to results
from new three-dimensional magnetohydrodynamic simulations. Special attention
is paid to the physics of faculae and to new three-dimensional radiation
magnetohydrodynamic simulations of the integral layers from the top of the
convection zone to the mid-chromosphere.
  The second part is devoted to a few basic properties of magnetic flux tubes,
which can be considered to be an abstraction of the more complicated flux
concentrations known from observations and numerical simulations. We treat
electrical current sheets, the mechanical equilibrium condition at magnetic
interfaces, the equations for constructing a magnetohydrostatic flux tube
embedded in a gravitationally stratified atmosphere, the condition of radiative
equilibrium, and the condition for interchange stability.
