Vocabulary and Writing in a First and Second Language is based on a large-scale empirical study. The innovative feature of the research was that the same students were asked to do the same tasks in both languages while reporting their thinking as they went along. Furthermore , they had to undertake the same tasks even though they were of very different experience, ranging from young children at school to university students. Three areas of learners' competencies and skills were explored: vocabulary knowledge, word guessing strategies and writing. The authors further explore the relationship between the skills and describe the level of development for individual learners within the three areas. In all cases, statistical and qualitative analyses are offered, the latter being based on the learners' own 'think-aloud' reports. Both researchers and teachers of language will find this in-depth approach useful in understanding the processes of both first and second language performance.