Processability Theory (PT) as developed by Manfred Pienemann is a prominent theory of second language acquisition. PT serves as a framework for a wide range of research covering issues, including L2 processing, interlanguage variation, typological effects on SLA, L1 transfer, pidgins and creoles, linguistic profiling, stabilisation/fossilisation and teachability. This textbook provides a reader-friendly introduction to PT. It is designed for students with a basic knowledge of (applied) linguistics. The components of PT are set out in four parts. The first part focuses on observed facts, in particular on paths of L2 development and learner variation. The second part gives an overview of the theoretical basis of PT. Part three details the application of PT to contexts other than ESL (i.e. Japanese, creoles and bilingual acquisition), and the forth part focuses on practical applications. Each chapter contains exercises (including data analysis and interpretation) which may be used for individual study or in class. The textbook can be used as a concise introduction to PT. However, it may also serve as a point of reference for particular PT-related topics. The individual chapters were written by specialists in each of the research areas.