Broad question. Do you mean domestically? Entering which country? Which country are you a citizen of? Etc.
Entry requirements vary widely from country to country and can depend on several variables: how long you plan on staying in a place, your 'regular' profession, information on the landing card, proof that you can support yourself while in a country and handle the questions from an immigration officer. Most countries want to make sure that your not having a return ticket doesn't equate to your staying in the country to work, etc. In the past I carried a copy of a recent bank statement or proof of a hostel reservation in an ongoing country I'm visiting to show my intention that I am just traveling through.
Best approach is to look at the State Department or equivalent website of the country(ies) you are planning to visit to see what they require for entry. almost 7 years ago