If you use other people’s material in your research you should be aware of both copyright and good research practices.

As a general rule the author (and their heirs) owns the rights to a work for 70 years after the death of the author/originator. After this, the work is no longer protected by copyright and can be used freely, although the author/originator must always be credited.

Texts

You are, of course, permitted to quote from texts. However, if you need to reproduce larger extracts you must obtain permission from both the author and usually the publisher as well.
You can read about quotations and correct source referencing at Forskerportalen (the Researcher Portal).

Pictures and illustrations

When you want to use other people’s pictures, drawings, photographs, illustrations, graphs, figures, tables, models etc. in your research publication please be aware that you must obtain the permission of the author/originator and, in most cases, the publisher’s permission as well.
Read more about obtaining permission to reproduce illustrations at the Copyright Clearance Center.

Photographs of works

When you want to use photographs of pictures, paintings, designs, architecture, theatrical production, and other works you must be aware that photographs are protected by copyright. Hence, you must ensure permission to include these in your work. This also applies if the photographed object is no longer protected by copyright law.
For more information on reproducing visual art and photographs, see the Ministry of Culture Denmark’s copyright page (in Danish)

Podcasts, audio and films

If you need to quote stills from a film or very short audio and film clips, such as those from performances, music, films, podcasts etc., you are allowed to do so. You can read more about the rules for quoting stills from films in UBVA’s answer bank (in Danish).

If you wish to reproduce more than very short clips you must obtain permission from the copyright holders, as music and films, computer games, etc. are protected by copyright.
Read more on the copyright page on the Ministry of Culture Denmark’s website (in Danish).

In order to republish your own texts or illustrations in a new context, make sure you have permission to do so from the publisher. Read more about republishing your publications.