Context. There is a need for developers to write clean code and code that adheres to a high-quality standard. We need developers not to introduce technical debt and code smells to the code. From a business perspective, developers that introduce technical debt to the code will make the code more difficult to maintain, meaning that the cost for the project will increase.

Objectives. The main objective of this study is to gain an understanding about the perception the developers have about clean code and how they use it in practice. There is not much information about how clean code is perceived by developers and applied in practice, and this thesis will extend the information about those two areas. It is an effort to understand developers' perception of clean code in practice and what they think about it.

Realization (Method). To understand the state-of-the-art in the area of clean code, we first performed  literature review using snowballing. To delve into developers' perception about clean code and how it is used in practice. We have developed and sent out a questionnaire survey to developers within companies and shared the survey via social networks. We ask if developers believe that clean code eases the process of reading, modifying, reusing, or maintaining code. We also investigate whether developers write clean code initially or refactor it to become clean code, or do none of these. Finally, we ask developers in practice what clean code principles they agree or disagree with. Asking this will help identify which clean code principles developers think are helpful and which are not.

Results. The results from the investigation are that the developers strongly believe in clean code and that it affects reading, modifying, reusing, and maintaining code, positively. Also, developers do not write clean code initially but rather refactor unclean code to become clean code. Only a small portion of developers write clean code initially, and some do what suits the situation, while some do neither of these. The last result is that developers agree with most of the clean code principles listed in the questionnaire survey and that there are also some principles that they discard, but these fewer.

Conclusions. From the first research question, we know that developers strongly believe that clean code makes the code more readable, understandable, modifiable, or reusable. Also, developers check that the code is readable using code reviews, peer reviews, or pull requests. Regarding the second research question, we know that developers mostly refactor unclean code rather than write clean code initially. The challenges are that to write clean code initially, a developer must have a solid understanding of the problem and obstacles in advance, and a developer will not always know what the code should look like in advance. The last research question showed that most developers agree with most of the clean code principles and that only a small portion of developers disagree with some of them. Static code analysis and code quality gates can ensure that developers follow these clean code practices and principles.