  This paper provides an overview and introduction to the development of
non-ergodic ground-motion models, GMMs. It is intended for a reader who is
familiar with the standard approach for developing ergodic GMMs. It starts with
a brief summary of the development of ergodic GMMs and then describes different
methods that are used in the development of non-ergodic GMMs with an emphasis
on Gaussian Process (GP) regression, as that is currently the method preferred
by most researchers contributing to this special issue. Non-ergodic modeling
requires the definition of locations for the source and site characterizing the
systematic source and site effects; the non-ergodic domain is divided into
cells for describing the systematic path effects. Modeling the cell-specific
anelastic attenuation as a GP and considerations on constraints for
extrapolation of the non-ergodic GMMs are also discussed. An updated unifying
notation for non-ergodic GMMs is also presented, which has been adopted by the
authors of this issue.
