To get to Phnom Penh, most travellers arrive by air or by bus; Cambodia has no rail network to speak of apart from a recently-opened line between Phnom Penh and Kampot.
Phnom Penh is served by one airport, Pochentong, which operates international and domestic flights. Domestic flights are available from Siem Reap and Sihanoukville. International flights are available fairly cheaply from the neighbouring countries using airlines such as Air Asia. The airport is located to the west of the city, roughly a 30-minute taxi or tuk-tuk ride from the city centre.
Alternatively, Phnom Penh can be reached by bus using a number of bus companies which operate day and night. Giant Ibis is a reputable bus company, but there are plenty of others. The main bus station is located on the Riverside near the Night Market, and a crowd of tuk-tuk drivers and motorcycle taxi drivers (motodops) usually enthusiastically accost arriving travellers stepping off the bus.