In a PhD or master thesis or dissertation, a literature review is generally placed at the beginning of your work, right after the introduction section. This varies according to university thesis format, but the literature review often comes at the beginning. It is an overall summary and criticism of all of the existing studies that are related to your research area. This section aims to provide a discussion of where the current research area stands to date, all of the related works related, and a critical evaluation of each work mentioned.
The research gaps or problems inherent in the works you have mentioned should be explicitly highlighted and discussed at the end of the literature review. This gives your readers an idea about the related works, the problems associated with them, and how your work aims to address these problems. Typically, you should aim to include recent works, but sometimes it is also important to include early foundations works. You need to mention that the foundation works are included to discuss a certain point, in order to avoid being criticized by readers of including outdated work.
The literature review is about 25-30% of the entire thesis, and is usually the largest sections in it. The works included should only be those that are within the scope of your research area, and the top few works that are closest to your work should be the primary focus. Be sure to remember that this is a synthesis and analysis. So it shouldn’t simply summarize the related works, but also criticize, evaluate and analyze them, while explicitly highlighting the gaps associated with them.
At the end of the review, it is common to include a recap of the important works, the problems associated with them, and how your work will address these problems. Simply summarizing the list of related works is insufficient, and does not form a complete literature review. Generally, a good review would show that you are familiar with your research area and topic, help you to highlight all of the problems inherent in prior work to form your problem statement, and demonstrate briefly how you plan to carry our your methodology to address the problem statement.
Searching for related studies in the body of literature can be tedious. But it can be simplified by using a list of the most common key words, as well as Boolean search operators, against the prominent scientific research databases available such as Scopus and Web of Science. Once done with compiling the list of related works, it is recommended to read each one thoroughly, but at the same time write down key concepts. This way you would be writing while the ideas you have read are still fresh in your mind. If you start to after you read all of the papers, however, you would have forgotten the first few papers and would need to re-read them. So writing a draft while reading is easier and this way you would end up with a nice draft of notes after you have read all of the works. You can then refine this draft and convert it into a well-written literature review.
Once complete with the review, you would need to carefully proofread and edit it in order to ensure it contains no grammatical errors and is easy to follow by your readers. The editing process also involves making it coherent where the flow of ideas is clear from paragraph to paragraph, and restructuring sentences to they are clear and concise.