Abstract Acquiring communicative competence and the ability to communicate in writing are essential goals for second language learners and of the highest importance to achieving educational success.  Opportunities to express ideas in writing are essential for students’ language development. Learners therefore need to be encouraged to take the risk of making errors in order to be able to express ideas, thoughts and knowledge with enthusiasm. This thesis defines and investigates some important factors contributing to the development of communicative competence and performance in the context of Second Language Acquisition (SLA) research and Communicative Language Teaching (CLT).  The main aim of the study is to focus on how Swedish learners make use of their first language (L1) as a cognitive and strategic source in their written English communication.  Cross-linguistic influence, or transfer errors, from Swedish in the written English of high school students are taken from samples of writing from the Uppsala Learner English Corpus (ULEC).  The study evaluates the potential of transfer from the L1 to either facilitate or inhibit the communicative purpose, i.e., ‘getting the message across’. The most serious errors in the data were found to result in confusing, inappropriate or incomprehensible structures.  Finally, the study draws attention to some aspects of communicative language teaching and learning that teachers should be aware of in the development and assessment of students’ communicative ability in written performance. The study found that negative language transfer was found to affect communication to various degrees of seriousness but could at times also be considered a useful and necessary strategy for getting a message across.  Transfer errors were often idiosyncratic and were most frequent among males between 16-17 years-old enrolled in vocational programmes.  Prepositional transfer errors were the most common, however; incorrectly used lexical items, in particular, false friends and other vocabulary substitutions, as well as literal translations of phrases and idiomatic expressions potentially resulted in the most serious errors.  Word order errors also interfered substantially with the message in many cases.  On the other hand, grammatical transfer errors such as incorrect us of articles, pluralisation and verb tense generally did not change the meaning to any greater extent.  In fact, transfer errors can be communicative as long as the message gets across.