  The QCD transition from a hadronic to a quark-gluon plasma phase is a
cross-over at vanishing/small baryo-chemical potential, while at higher
chemical potentials it is argued that it becomes of first order, ending with a
critical end-point. The present goal is the determination of the critical line
and, possibly, the recognition of the critical endpoint. For this purpose, the
effective model (Nambu-Jona-Lasinio model with Polyakov Loop) with 2+1 flavours
is used. In Mean Field Approximation it is possible to obtain all thermodynamic
quantities (pressure, entropy, energy and quark density) and their
fluctuations, in particular, the generalized quark-number susceptibilities. We
use the net baryon-number fluctuations and net-strangeness fluctuations to
identify the critical line and the order of the phase transition in the
chemical potential - temperature plane.
