There are two ways by which you can travel to Bhutan: Either by Drukair or overland through Phuentsholing, a small town in south Bhutan bordering Indian state of West Bengal. Drukair, the only national airline of Bhutan services to and from Bangkok, Delhi, Kathmandu, Kolkotta, Dhaka and Singapore. Quite recently, another locally owned airline, Tashi Air has been added to the Bhutanese sky.
It operates to and from Bangkok, Kolkotta, Kathmandu and Delhi. Overland entry from Phuentsholing is more convenient if you wish to tour Sikkim and Darjeeling before your tour in Bhutan. While you can exit from Samdrup Jongkhar, a town in south east of Bhutan bordering Assam. This exit is more convenient if you wish to tour Assam after Bhutan without tracing back the same lateral route back to Paro for your flight to other destinations.