  The Cluster HEritage project with XMM-Newton - Mass Assembly and
Thermodynamics at the Endpoint of structure formation (CHEX-MATE) is a three
mega-second Multi-Year Heritage Programme to obtain X-ray observations of a
minimally-biased, signal-to-noise limited sample of 118 galaxy clusters
detected by Planck through the Sunyaev-Zeldovich effect. The programme,
described in detail in this paper, aims to study the ultimate products of
structure formation in time and mass. It is composed of a census of the most
recent objects to have formed (Tier-1: 0.05 < z < 0.2; 2 x 10e14 M_sun < M_500
< 9 x 10e14 M_sun), together with a sample of the highest-mass objects in the
Universe (Tier-2: z < 0.6; M_500 > 7.25 x 10e14 M_sun). The programme will
yield an accurate vision of the statistical properties of the underlying
population, measure how the gas properties are shaped by collapse into the dark
matter halo, uncover the provenance of non-gravitational heating, and resolve
the major uncertainties in mass determination that limit the use of clusters
for cosmological parameter estimation. We will acquire X-ray exposures of
uniform depth, designed to obtain individual mass measurements accurate to
15-20% under the hydrostatic assumption. We present the project motivations,
describe the programme definition, and detail the ongoing multi-wavelength
observational (lensing, SZ, radio) and theoretical effort that is being
deployed in support of the project.
