Depending on where you are traveling from, but usually all you need is a valid passport. If you are EU citizen, then just valid ID will be sufficient. In case you come from some of these countries, you will have to obtain a visa to enter to Croatia: Algeria, Armenia, Azerbaijan, Bahrain, Belarus, Egypt, Philippines, India, Indonesia, Jordan, South Africa, Qatar, Kazakhstan, China, Kosovo, Kuwait, Lebanon, Morocco, Mongolia, Nigeria, Oman, Russia, Saudi Arabia, Thailand, Turkey, UAE, Ukraine and Vietnam.