Diverse modeling and simulation methods are being applied in the area
of systems biology.  Most models in Systems Biology can easily be
located within the space that is spanned by three dimensions of
modeling: continuous and discrete; quantitative and qualitative;
stochastic and deterministic. These dimensions are not entirely
independent nor are they exclusive. Many modeling approaches are
hybrid as they combine continuous and discrete, quantitative and
qualitative, stochastic and deterministic aspects.  Another important
aspect for the distinction of modeling approaches is at which level a
model describes a system: is it at the ``macro'' level, at the
``micro'' level, or at multiple levels of abstractions.  Many challenges for modelling and simulation methods arise from this muli-level and multi-formalism modeling and simulation.