The present thesis consists of two parts. First, I am going to describe dictionaries generally, including what they are at all, what they include, what they are used for, who uses them and what the general principles are to be taken into consideration on the part of lexicographers, considering both printed and online dictionaries. In the second part of the thesis I am going to investigate online dictionaries. First, the monolingual English ones, then the bilingual English-Hungarian ones.