  This is a survey on the mixed spin P-fields (MSP fields for short) theory
which provides a platform to understand the phase transition between
Gromov-Witten theory of quintic CY 3-folds and Landau-Ginzburg theory of the
corresponding quintic polynomials. It discusses key ideas that lead to the
definition of MSP fields and how moduli of stable maps to the quintic and that
of 5-spin curves appear in the moduli of MSP fields. It also explains some
properties of the moduli of MSP fields such as the cosection localisation, the
properness of the degeneracy locus, and a torus action on the moduli.. Some
vanishings arising from the torus action provide polynomial relations among
GW-invarants and FJRW-invaraints which give an effective algorithm for the
computation of those invariants. Some examples of computations of genus 1 low
degree of GW invariants are provided.
