Thomson scattering (TS) is a proven diagnostic technique that will be implemented in ITER in three independent systems. The Edge TS will measure electron temperature T e and electron density n e profiles at high resolution in the region with r/a>0.8 (with a the minor radius). The Core TS will cover the region r/a<0.85 and shall be able to measure electron temperatures up to 40 keV. The Divertor TS will observe a segment of the divertor plasma more than 700 mm long and is designed to detect T e as low as 0.3 eV.The Edge and Core systems are primary contributors to T e and n e profiles. Both are installed in equatorial port 10 and very close together with the toroidal distance between the two laser beams of less than 600 mm at the first wall (∼ 6 • toroidal separation), a characteristic that should allow to reliably match the two profiles in the region 0.8 < r/a < 0.85.Today almost every existing fusion machine has one or more TS systems installed, therefore substantial experience has been accumulated worldwide on practical methods for the optimization of the technique. However the ITER environment is imposing specific loads (e.g. gamma and neutron radiation, temperatures, disruption-induced stresses) and also access and reliability constraints that require new designs for many of the sub-systems.The challenges and the proposed solutions for all three TS systems are presented. K : Plasma diagnostics -interferometry, spectroscopy and imaging; Plasma diagnosticscharged-particle spectroscopy 1Corresponding author.