The main aims of the thesis are: to explore the development of Swedish university students’ vocabulary knowledge, size and depth and vocabulary use (i.e. lexical richness) in their written output, to examine the relationship between these, their relationship to examination grades, and to investigate how these relationships develop over time. The results showed that over one year of university studies stronger links between the two dimensions of vocabulary knowledge, size and depth are established. No relationship was found between informants’ vocabulary size and lexical richness. However, a modest relationship was found between depth and the lexical richness of student essays. Furthermore, there was a modest relationship between vocabulary knowledge and academic performance. A weaker significant, relationship was found between lexical richness of student essays and academic performance as reflected in the course grade. However, the study did not show evidence of a relationship between lexical richness and essay grade, which seems to indicate that lexical richness, is not an essential criterion in teachers’ assessment of essays. In regard to the development of the informant’s vocabulary knowledge, there was a significant growth in their productive size and depth of vocabulary knowledge after both one and two terms. The informants’ receptive size was found to only develop over two terms of study. Moreover, they produced lexically richer essays in their second term than in their first term of study. The results of the study are discussed in light of the effect of similar learning experience at university and the onset vocabulary ability of the informants. Moreover, the findings are discussed from the perspective of pedagogical implications and vocabulary assessment.