  This paper introduces an intrinsic theory of Thermodynamic Formalism for
Iterated Functions Systems with general positive continuous weights (IFSw).We
study the spectral properties of the Transfer and Markov operators and one of
our first results is the proof of the existence of at least one
eigenprobability for the Markov operator associated to a positive eigenvalue.
Sufficient conditions are provided for this eingenvalue to be the spectral
radius of the transfer operator and we also prove in this general setting that
positive eigenfunctions of the transfer operator are always associated to its
spectral radius.
  We introduce variational formulations for the topological entropy of
holonomic measures and the topological pressure of IFSw's with weights given by
a potential. A definition of equilibrium state is then natural and we prove its
existence for any continuous potential. We show, in this setting, a uniqueness
result for the equilibrium state requiring only the G\^ateaux differentiability
of the pressure functional. We also recover the classical formula relating the
powers of the transfer operator and the topological pressure and establish its
uniform convergence. In the last section we present some examples and show that
the results obtained can be viewed as a generalization of several classical
results in Thermodynamic Formalism for ordinary dynamical systems.
