The online knowledge testing and evaluation pose one of the main parts in the area of distance education. Their greatest problem is plagiarism. The authors deal with the term definition of the plagiarism and go into some important facets of the plagiarism prevention and revelation in details. They analyze actual state of the fight against plagiarism in Slovakia and in other countries. They try to classify the kinds of online testing and evaluation activities that are supported in contemporary LMSs in the second part of the paper. They describe their own experience in plagiarism reducing in computer science e-learning courses. Finally, the authors stress out the important role of plagiarism prevention and bring several methods that could reduce the risk of plagiarism in e-learning.