This paper presents an online course for assessing and developing lexical competence which grew out of one simple question: How many words do our students have? We start by highlighting the importance of vocabulary teaching in the second language learning process and the beneficial effects of multimedia on developing lexis. The first response to the aforementioned query took the shape of a research study which evaluated the lexical competence of students in their final year of an English Philology degree course. As a result, the lecturers involved in the project undertook a further study entitled ADELEX – ‘Assessing and Developing Lexical Competence through the Internet’ – aiming to develop a web-based course that would improve the vocabulary levels of our students (http://www.ugr.es/~inped/index.htm). The instructional design of the online vocabulary project, ADELEX, developed through Web Course Tools (WebCT), is detailed in the second part of the paper. First we set out its pedagogical objectives and describe the main characteristics of this modular course. We then address the issue of syllabus design describing its lexicographical, lexicological, semantic and discoursal contents, and offer a brief profile of its nine modules. Needless to say, we establish some methodological guidelines for the implementation of a virtual course which may help students control their own learning and provide constant feedback and interaction. In fact, learner autonomy is one of its main objectives. In addition, its teaching procedures are examined, emphasizing the tools of WebCT: online assignments, forums, online vs. on campus sessions, students’ projects, grading, test design, etc. An unavoidable research question is also addressed: how to assess lexical competence both at the diagnostic level, to obtain initial feedback, and at the achievement level, i.e. when a word can be said to be acquired in a web-based environment. The answer to this question has been to develop a complex system of computer-based tests (CBTs). Finally, we draw some pedagogic conclusions.