United States copyright law recognizes that there are certain uses of copyrighted works that benefit society without causing undue harm to copyright holders. These kinds of uses are allowed under the doctrine of fair use, which means they do not require permission from the copyright holder.
Some examples of fair use listed in copyright law include criticism, commentary, parody, news reporting, teaching, scholarship, and research. However, not all uses made for these purposes will automatically be fair. Instead, copyright law provides a framework to determine what uses should be considered fair. This means that fair use must always be considered on a case by case basis.
The fair use framework is purposefully vague so that it can remain flexible as technologies and types of uses change over time. However, this can make it more difficult to decide whether or not your use falls under fair use. In order to make this decision, you must analyze the particular details of your situation and proposed use.
Everyone is free to make fair uses of copyrighted works, but copyright owners can disagree on what qualifies as fair use and assert that uses are copyright infringement. This means that those who want to exercise their fair use right should consider their use under the framework provided by copyright law and weigh the risks involved in certain uses.
For examples of cases that courts have decided as either fair use or not fair use (copyright infringement), see:
Copyright and Your Dissertation or Thesis: Ownership, Fair Use, and Your Rights and Responsibilities by Kenneth Crews is a must read resource for graduate students as they prepare their thesis or dissertation. In it, you will find information about fair use as it specifically applies to theses and dissertations, including a more detailed breakdown of the four factors and examples of fair uses.
As you consider your uses of others' works, you may also find it helpful to refer to various statements of best practices that describe situations considered to be fair use of copyrighted works by various scholarly communities. While a statement of best practices might not exist for your specific discipline, these statements may help guide your thinking, as there are many scholarly uses that are considered fair across disciplines.
You might also find these tools helpful when you are analyzing your proposed use: