Can a U.S. citizen enter the u.s. without a Passport?

A US passport is not required for US citizens entering the US by air, land or sea. However, US citizens are required to show proof of citizenship such as a birth certificate or passport card when entering the US by land or sea. US citizens flying into the US from another country are required to show a valid passport. So, while a US passport is not required for all US citizens entering the US, it is recommended that all US citizens travelling internationally carry a valid US passport.