The most convenient international bus service into Namibia runs from Cape Town and Victoria Falls. There is also service from Johannesburg. See Intercape Mainliner (http://www.intercape.co.za) for schedules and fares. Using a combination of buses, hitchhiking and kombis you can also get to Namibia from anywhere in Botswana.
Hosea Kutako International Airport, located 45 minutes east of Windhoek, is the main entry point for air traffic. Air Namibia (http://www.airnamibia.com) operates flights from Frankfurt, Cape Town, Johannesburg, Victoria Falls and Maun to the international airport. Flights between the smaller Eros Airport and Cape Town are also available. South African Airways (http://www.flysaa.com) and no-frills Kulula.com (http://www.kulula.com) operate flights from South Africa, too. See Discount airlines in Africa for more options.
The regular overnight train from Upington in South Africa to Windhoek, operated by TransNamib (http://www.transnamib.com...), has been discontinued. It is no longer possible to get into or out of Namibia by train.