This crosslinguistic collection looks at changes and developments in language known as progression and regression. The processes are examined and compared at the societal and individual level. In the former, the focus is on the socio-cultural forces that influence groups of speakers to create new languages or abandon old ones. In the latter, the acquisition and attrition of first and second languages are considered. The theoretical issues around this topic are addressed from sociological, neuropsychological and linguistic perspectives.