The purpose of this thesis is to establish and describe how often, in what places, and why Swedish forum members code-switch to English. This is achieved by examining 3,600 posts (180 posts from 20 different threads) belonging to five categories on the biggest Swedish forum, Flashback. The framework for the analysis of the code-switched instances is not taken from just one source, but is rather a mixture of different sources, each of which focuses on different facets which are usually observed when it comes to code-switching. However, it should be mentioned that some new categories have been added so as to achieve a higher degree of precision when analyzing the examples. The thesis examines the grammatical, structural and functional aspects of code-switching with the aim of establishing the type of switching employed, the type of English unit which has been inserted into an otherwise Swedish (written) speech, its word class and the potential reasons why the forum members opted to switch to English. The thesis also takes a closer look at some of the debates which bear significant relevance when it comes to determining the criteria for what should be considered code-switching and what should not, such as code-switching vs. (nonce) borrowings and the debate surrounding intra-word switching. It also offers some information about the status of English in Sweden and describes some of the advantages and disadvantages of examining code-switching in online communication and on forums. After the 20 threads have been examined and each example of code-switching analyzed, it can be concluded that Swedish forum users tend to use single word switching in more than half of the 628 cases of code-switching which have been recorded. The most commonly employed word class are nouns, which occur in more than 40% of the cases and the forum members do not frequently plunge into creating their own random series of words, clauses and sentences on the spur of the moment, but rather stick to the well-established phrases, clauses and sentences which are more often than not rather simple. When it comes to the reasons for switching, in many cases they feel that using an English switch is a more appropriate way to talk about a particular topic, they also employ them frequently for in-group communication, to emphasize something, out of a habit or for the sake of convenience, because of a lexical need and for other reasons. Probably the biggest takeaway from this analysis is that there are more than twice as many code-switched instances in the five threads that are related to an environment, culture or a situation in which English is mainly spoken, as opposed to the five threads that are related to Sweden and its environment, culture and language. This could mean that code-switching might occur more frequently because of an unconscious influence that the English-related threads wield over the Swedish forum members when they discuss such a thread.