Bhutan is a landlocked country. You can travel to Bhutan either by flight or overland.

Getting into Bhutan (Air)
Bhutan has only one international airport in Paro that caters to the needs of visitors. Paro international airport is located at a height of 7,300ft and surrounded by mountains and hills as high as 16,000 ft.
Only two national commercial airlines operate and land in Bhutan, DrukAir and Bhutan Airlines. These two airlines have routes to Delhi(India), Kolkota(India), Bagdora(India), Guwahati(India), Dhaka(Bangladesh), Changi(Singapore), Bangkok(Thailand), and Katmandu(Nepal).
Flying to Bhutan & landing in Paro is an adventure in itself. Paro International is listed as one of the most difficult airports to land-in. But the adrenaline, and the mesmerizing view of the mountains during flight make it a worth-to-fly flight.
Please note that due to variable weather conditions, and the fact that Paro Airport can operate only during day, flight delays can occur. It is advisable to keep more layover time between your exit flight from Paro and the next flight from your onward destination.

Getting into Bhutan (Land)
You can enter/exit Bhutan from three different roads connecting to India. Phuentsholing, Gelephu and Samdrup Jongkhar are the only land border areas open to tourists.
The town of Phuentsholing is located approximately 170 km from Bagdogra international airport in Siliguri. From Siliguri to Phuentsholing it takes about 4 hours to reach Jaigoan (Bhutan – India border). After crossing Phuentsholing, you begin your journey to Thimphu, the capital city takes about six hours (170 km).

Gelephu in south-central Bhutan is another entry point to Bhutan. It is approximately 250 km from Thimphu and the journey will take you through the sub-tropical areas of Bhutan before entering the alpine zone and then finally into Thimphu. One will have to traverse across three districts and the travel time will be about ten hours. Alternatively, one may choose to take a domestic flight from Gelephu to Paro.

The district of Samdrup Jongkhar in southeast Bhutan borders the Indian district of Darranga, Assam and is approximately 150 km away from Guwahati, the capital city of Assam. The journey from Guwahati is about three hours.