The Tribhuvan airport in Kathmandu is Nepal's only one international airport. The important airlines that serve Kathmandu are Nepal Airlines, Indian Airlines, Thai Airlines, Bangladesh Biman, China Southwest Airlines, Druk Air, Qatar Airways, PIA- Pakistan Airlines, Gulf Air, Sahara Airlines, Jet Air, Himalayan Airlines, Indigo Airlines, Oman Airlines, Turkish Airlines and Cosmic Airlines.
Since it has India on three sides and China to the north, traveling to Nepal by land requires that one pass through either one of these nations. The entry points from India are Biratnagar, Kakkarbhitta, Birgunj, Bhairawa, Nepalgunj, Dhangadi and Mahendra Nagar. The Kodari pass on the Nepal-China border is the entry point from China. Tourists entering the erstwhile kingdom by land must possess an international carnet.
You could travel to Kathmandu via Bangkok, Hong Kong, and Singapore. There are daily flights to Kathmandu from Bangkok.If in India, you can fly to Nepal from Delhi, Mumbai, Calcutta, Banglore, and Varanasi. There is the spectacular flight from Lhasa to Kathmandu on Saturdays, Tuesdays, and Thursday operated by China Southwest Airlines. You can also fly Druk Air from Paro in Bhutan or take a flight from Dhaka, Bangladesh.
The crossing points from India include Mahendranagar, Dhangadhi, and Nepalgunj in the west, Sunauli, Birgunj, and Kakarbhitta in the east. Make sure to book your tickets through a reputed agency to avoid getting duped. Also, bear in mind that everyone has to change buses at the border whether they book a through ticket or not and that despite claims to the contrary.
The crossing points from Tibet is Tatopani friendship Bridge. There are others crossing points from Tibet as Upper Upper Mustang, Dolpo, Simikot, and others normally Tatopani friendship bridge is main business crossing points. It also points to visit Tibet Lhasa, Mt. Kailash exit from Tibet to Nepal. There is also a flight to Tibet and Tibet to Kathmandu.
From Europe and the Middle East
Qatar Airways and Gulf Air operates daily flights to Kathmandu from Doha and Dubai Pakistan International, Bangladesh Biman and Aeroflot have one-airline service from Europe to Kathmandu. The other option is the charter companies, Austrian Airways has a direct flight from Vienna to Kathmandu and Martin Air operates a direct flight to Kathmandu from Amsterdam.
From North America
You could fly to Nepal via India or alternatively via Bangkok or Hongkong. NAC's Osaka flight makes good connections with North American flights.