When planning a new power system, in particular at a new voltage level, insulation coordination (IC) is one of the most important subjects. The main task is the determination of stresses and the assessment of the strength of the system and the equipment installed. The general procedure of IC is described in IEC 60071. However, this standard does not give precise advice regarding new voltage levels at & above 1000 KV. However, experience is available from former CIGRE work which also considered the 1000 kV voltage level. The lightning and switching overvoltages are of particular interest regarding stresses. For determination of overvoltages the well known models and methods from various CIGRE reports are applied & corresponding examples and simulations will be presented. The overvoltages in UHV system like Temporary, Slow front, Fast front & Very fast front overvoltages are considered & dielectric strength of insulation at operating voltage, switching impulse, lightning impulse & very fast transient stress will be examined & basic design of Insulation coordination for UHV (of & above 1000 KV) is suggested. In addition, special consideration is made on the performance of SF6 insulating systems which will frequently be applied in the voltage range in question.