As the most massive gravitationally-bound objects in the Universe, galaxy clusters represent fundamental signposts in the story of structure formation and evolution.

The distribution of their gravitating and baryonic mass is the key ingredient to use galaxy clusters as astrophysical laboratories and cosmological probes.

We propose an international team of experts working on the X-ray and lensing properties of galaxy clusters to meet at ISSI to share their common experience on this topic, focusing on the following items:

1. (1)the reconstruction of the cluster mass profiles through
   

   X-ray, strong and weak lensing techniques;

2. (2)the use of X-ray and lensing derived quantities as proxies of
   

    the gravitating mass;

3. (3)mapping of the cluster outskirts with X-ray and weak
   

     lensing methods;

4. (4)estimates of the systematics affecting mass reconstruction and cosmological implications of these measurements.
   

Taking benefit from the opportunity provided from ISSI to support both logistically and financially meetings in which these issues can be discussed, we plan to produce documents that takes stock of these research fields and to promote activities that allow the international community to make significant advances on this topic through conferences, workshops and applications for funding.