U.S. citizens: Passports are still required. Please check your passport to make sure it's still valid.
Non-U.S. citizens: Canada has a relatively new (8/15) entry requirement known as the Electronic Travel Authorization (eTA) for visa-exempt foreign nationals travelling to Canada by air. Exceptions include U.S. citizens and travelers with a valid visa. Entry requirements for other methods of travel (land, sea) have not changed. Eligible travelers can apply online for an eTA.
If you do not have an eTA, you will need to contact the Canadian consulate, high commission or embassy for information before you leave your home country. Find out what documents you will need to enter Canada and check if there are any recent changes to Canadian immigration policies. You should determine whether or not you require a temporary visa and if you do, the cost.