Grading - As mentioned above, English-English dictionaries are available in many different levels. By using an Intermediate Learners' Dictionary, for example, you will know that any words which you read in a text that do not appear in the dictionary are not important enough to learn at this stage of your English studies and so can be ignored. If there are many words in the article you are reading that are not in the dictionary for your level, that is also a good sign that the text is too difficult and you should reading a graded text instead. Bilingual dictionaries are not often graded this way.