Background: The phenomenon of code-switching has received some attention, but there are still areas where it can be developed. The prevalence of multilingual people in Sweden and within the Swedish school system is higher today than in previous decades. Which leads to the topic of code-switching and how it takes place in formal forms such as in school and during lessons.

Aim: This study aims to investigate and identify the functions behind code-switching in an English classroom with multilingual students in a Swedish upper secondary school.

Method: Data were collected through classroom observations and semi-structured interviews. To be able to analyse and identify the various functions of code-switching, the Myers-Scotton Markedness Model was used as a theoretical frame of reference.

Conclusion: It is clear that code-switching does not slow down communication, but rather allows it to flow, which in itself seems to enhance learning. By teachers allowing their students to code-switch when they have difficulties finding the right word and thus allowing the conversation to continue despite switching to another language the learning and lecture can move forward. The research also identified multiple factors and functions behind code-switching by the use of the Markedness Model.