The dynamics of development and the complexity of systems involved in second language learning necessarily imply some form of interplay between the systems in action. As has been described earlier in this book this interconnection is supposed to proceed in accordance with the Trade-Off Hypothesis proposed by Skehan (2009), which suggests that all three proficiency dimensions, i.e. Complexity, Accuracy and Fluency, do not develop in parallel and focusing more on one of them at the expense of the other two will weaken the performance of the latter. In other words: second language learners are unable to handle all three simultaneously and when they pay more attention to, for example, fluency they will automatically concentrate less on accuracy and complexity, which may lead to a situation where they will speak quickly but at the same time their texts will be rather simple and contain many errors. When we base our observations on the averages for each dimension and look at the linear correlation between them (see Table 8.1) the interplay between all three cannot be ignored. In each case the correlation is at least moderately strong with the lowest interconnectedness between accuracy and syntactic complexity (subordination ratio) r =. 58 and the highest between fluency (rapidity measured as the number of words written per minute) and syntactic diversity (number of different clauses per sentence) r =.92. When it comes to the interplay between complexity, accuracy and fluency for the group as a whole we could reject the Trade-Off Hypothesis. The outcomes presented below tend to confirm the continuing interplay of all systems and provide proof that the development of these proficiency dimensions generally occurs simultaneously. However, a more in-depth analysis of the data reveals an interesting regularity. Although in all three cases the correlation is strong or moderately strong a common pattern can be observed, namely that in general interconnection is weakest between accuracy and syntactic complexity (both in terms of the diversity of syntactic structures and the ratio of subordinated clauses), as well as between fluency and lexical complexity, i.e. the ability to connect words in more complex phrases.