It is best to have a valid passport that will not expire within six months of the date you intend to leave Uganda. Should your passport get stolen or lost while in Uganda, it is generally easier to get a replacement if you have a photocopy of the important pages.
Nationals of most countries need a visa to enter Uganda and although previously it was better to buy these well in advance at any Ugandan embassy or High Commission abroad, it’s simpler to buy the visa upon arrival and much as some queuing is inevitable, it’s a straightforward procedure and cheaper since one gets there visa there and then of course after they have queued.
A standard single entry visa valid for three months costs US$ 50, student visa’s cost US$ 100 and multiple entry visa’s (6-12 months) cost US$ 100, (24 months) US$150 and (36 months) US$200. Once in East Africa, a single entry visa for Uganda, Kenya and Tanzania allows you to visit either of the other two countries (when you’ve met their visa requirements) without and return without having to apply for another visa. However, this is different for Rwanda and Congo.
If you leave Uganda for these countries (say to track gorillas), you must purchase another visa on re-entry but if you intend to spend seven days in Uganda before leaving again, then you purchase a transit visa. Otherwise you will incur more for another visa and on top of that most foreign nationals require a visa to enter Rwanda which can also be bought on arrival at any border.