This thesis is placed in the context of the ongoing debate on the issue whether second language acquisition occurs on the basis of innate language-specific learning mechanisms or general learning mechanisms. The author shares the view of scholars who propose that an innate knowledge base underlying first language acquisition does not extend to second language acquisition due to the lack of uniform success in the acquisition of native speaker competence, the possibility of fossilisation and the facilitative potential of form-focused instruction. It is, thus, assumed that the sequential second language acquisition process can be accounted for by general learning mechanisms. The key to these learning mechanisms is, firstly, the investigation of the nature of the knowledge underlying the grammatical encoding procedures for speech production in the context of M. Pienemann’s (1998a) Processability Theory and, secondly, the investigation of the nature of its acquisition process. Pienemann’s Processability Theory explains and predicts the sequential acquisition process of a second language as the result of the hierarchically ordered development of the processing procedures of the grammatical processor to grammatically encode conceptual information. It shares with Levelt’s (1989) theory of speech production the assumptions concerning the nature of the knowledge underlying the grammatical encoding procedures, which require further investigations for verification. Since the Processability Theory does not specify how the assumed knowledge underlying grammatical encoding is acquired, an investigation of the nature of its acquisition process is necessary. This investigation highlights the interdependence between the nature of the knowledge to be acquired and the nature of its acquisition process by demonstrating that the knowledge underlying grammatical encoding is predominantly implicit and, consequently, determines the implicit nature of its acquisition process. Such implicit knowledge is dissociated from explicit knowledge, which determines the explicit nature of its acquisition process. This investigation also demonstrates that explicit grammar teaching and practice in the context of the manipulation of the learners’ attentional orientation mediated by alertness may contribute to the implicit learning process under certain conditions. In conjunction with the provision of guidance by the Processability Theory in regard to the achievement of instructional focus and the independent finding that comprehensible input is needed in order for second language acquisition to occur, these results constitute the basis for the formulation of detailed instructional measures for the effective organisation of the sequential second language acquisition process. These measures are exemplified by their implementation for the initial stages of the acquisition of German as a second language. / PhD Doctorate