An almost mandatory piece of software is a reference manager; we say almost because, although it would be possible to write a PhD without one, we highly recommend using one.  A reference manager allows you to save information on all of your research sources, journal articles, books, reports, etc., in one central location and export both citations and full bibliography in a variety of different referencing styles and formats, including the standard options required by all of the schools that make up Cambridge.

Three of the most common choices amongst students and researchers are EndNote (including its online form), Zotero, and Mendeley; a brief comparison between these three reference managers is provided in the table below.  There are other pieces of software available but they are less commonly used and/or popular.

Regardless of which reference manager you choose, you should make sure that it its desired features are compatible with the writing software you are using; for example, there is no direct plugin for transferring references from Zotero to macOS's Pages, even though it is relatively simple to copy references and citations to the clipboard, but this might be a feature you are expecting.  Do also check that you are comfortable with the method for outputting/inserting references according to the style required by your department, such as bibtex for latex.