The aim of this study is to discuss the interaction of linguistic complexity and morpho-syntactic development in foreign language learners. The analysis of morpho-syntactic structures was carried out within the framework of Processability Theory (Pienemann 1998). To capture the level of complexity we investigate the following: sentence length, subordination and nominal vs. verbal style. Oral and written data from four adult learners were collected over a period of one year. The analysis of complexity and morpho-syntax suggests that learners can be divided into different types, such as: The Risk-taker, The Recycler and The Careful and Thorough. (Contains 14 tables and 4 notes.) [Funding for this research was provided by The Swedish Research Council.]