62-year-old Singh, who was elected from Lucknow Lok Sabha constituency, succeeds Sushilkumar Shinde in the crucial Ministry that handles country's internal security and the task of guarding the international border with Pakistan, China, Bangladesh, Nepal and Bhutan.
The immediate issues that the teacher-turned politician will have to deal with include tackling the Naxal violence, activities of terrorist outfits in country's hinterland, infiltration from Pakistan, illegal immigration from Bangladesh and strengthening intelligence gathering mechanism.
As Home Minister, the two-term BJP president will be part of Prime Minister Narendra Modi-led Cabinet Committee on Security, the country's highest decision making body on security matters.
It was under Singh that Vajpayee's dream project, National Highway Development Programme was launched. He was also Education Minister in the Kalyan Singh government in Uttar Pradesh.
Singh, who taught physics at a college in Gorakhpur in his early career, has earned the reputation of being an able administrator as Union Minister and Chief Minister of Uttar Pradesh.