It is a common knowledge in the internet that images and media developed by the agencies of US Government such as NASA and NOAA are in the public domain unless stated otherwise. However, reading certain policies of these two agencies made me confused as to what constitutes "public domain". From NOAA Education Resources: From NASA GOES Project Science: From NASA Astronomy Picture of the Day: The underlined text in the quotation above is a hyperlink. If you access the official guidelines, it states: They have separate policy for using materials for advertising purposes. Ok! I know how to distinguish which images are actually owned by NASA/NOAA and which ones are owned by outside contributors who might retain the copyrights so let's stick to those that are actually owned by NASA/NOAA which according to some of its websites are in the public domain. Statements about the images being in public domain, and requiring credits or acknowledgement appear side by side. Isn't that contradictory? Isn't that when you say Public Domain, it means you can do whatever you want even for profit or not for profit? With or without acknowledgement from the original creator? I mean, why would NASA and NOAA use the term public domain to refer to their images when they are asking for acknowledgement at the same time. Why not just write "You can use our image provided you give us credit". It seems like the creations of US Federal Government are not really in the public domain. Anyone who can clarify this to me? Thank you and have a great day!