This study investigated lexical complexity in English language teaching (ELT) students' essays across genres and over the course of time. The essays included descriptive, compare and contrast, advantage and disadvantage, and argumentative genres. Employing a longitudinal, quasi-experimental and repeated measures design in a year-long writing course in the ELT department of a northeastern Turkish university, we analyzed lexical density, lexical sophistication and lexical variation in a corpus of 229 essays via the web-based lexical complexity analyzer developed by Ai and Lu (2010). The scores of all measures were analyzed using SPSS for further examination through descriptive and inferential statistics. Of the three indices, only the lexical density index was found to have a significant difference across genres, with the highest score in the descriptive essay. The findings related to lexical complexity development over time revealed that a statistically significant change was only captured in the lexical variation index. These findings might contribute to the language acquisition research as our study encompasses the examination of all three lexical complexity indices across four different genres and over time. In this vein, some considerable implications for future writing research and for writing instruction in foreign/second language education are suggested.