The paper reviews scientific and technological issues related to repair weldability of nickel base single crystal superalloys. The role of weld pool geometry and crystallographic orientation of the single crystals on the solidification grain structure was discussed. Results of asymmetric cracking during welding of thin crystals due to stray grain formation were introduced. The role of high resolution characterisation tools including electron microscopy and atom probe field ion microscopy in understangding non-equilibrium microstructure evolution was demonstrated. Computational methodologies to describe the dynamic thermal, stress and microstrutural evolutions to describe weldability as well as the performance of these welds were discussed.