Opened to foreigners in 1992, a travel permit is a must for them.
The bumpy and uncomfortable drive from Shimla and Manali towns onwards towards Spiti is a motorist's nightmare. The road remains closed at one place or another most of time.
So check on general road conditions and confirm whether the high mountain passes are open.
If driving in a four-wheeler, make sure you have an extra driver, especially if you plan to drive at night. Carrying extra fuel is a must.