Systemic Functional Linguistics is unique among linguistic theories in treating the concept of system as the central organising principle of language (and also of other semiotic systems, including context), most theories being focussed on syntagmatic structure. This book introduces the notion of system as the foundation of the systemic functional architecture of language, relating the general notion of system in systems thinking (holistic approaches) to the principle that language is organised as a system of systems (the polysystemic principle) and, by another step, to the technical sense of system in SFL as the basic category of paradigmatic patterning - i.e. the organisation of language as a resource for making meaning. The concept of system is then used to explore the emergence of complexity in language (within different semogenetic timeframes), to show how it is manifested in the organisation of all subsystems of language (the fractal principle), to illustrate the system at work in the development of language descriptions and in the process of text analysis, to reveal the power of the system in different areas of application, e.g. in computational modelling, in educational analysis and curriculum development, in multilingual and multimodal studies. Finally, challenges are identified e.g. in the relationship between the paradigmatic axis and the syntagmatic one, in the representation of logical iteration and interpersonal continua; and current and new opportunities are suggested.