For almost fifteen years, the ACL SIGWAC, and most notably the Web as Corpus (WAC) workshops, have served as a platform for researchers interested in the compilation, processing and use of webderived corpora as well as computer-mediated communication. Past workshops were co-located with major conferences on corpus linguistics and computational linguistics (such as ACL, EACL, Corpus Linguistics, LREC, NAACL, WWW). In corpus linguistics and theoretical linguistics, the World Wide Web has become increasingly popular as a source of linguistic evidence, especially in the face of data sparseness or the lack of variation in traditional corpora of written language. In lexicography, web data have become a major and wellestablished resource with dedicated research data and specialised tools. In other areas of theoretical linguistics, the adoption rate of web corpora has been slower but steady. Furthermore, some completely new areas of linguistic research dealing exclusively with web (or similar) data have emerged, such as the construction and utilisation of corpora based on short messages. Another example is the (manual or automatic) classification of web texts by genre, register, or – more generally speaking – “text type”, as well as topic area. In computational linguistics, web corpora have become an established source of data for the creation of language models, word embeddings, and all types of machine learning. The 12th Web as Corpus workshop (WAC-XII) looks at the past, present, and future of web corpora given the fact that large web corpora are nowadays provided mostly by a few major initiatives and companies, and the diversity of the early years appears to have faded slightly. Also, we acknowledge the fact that alternative sources of data (such as data from Twitter and similar platforms) have emerged, some of them only available to large companies and their affiliates, such as linguistic data from social media and other forms of the deep web. At the same time, gathering interesting and relevant web data (web crawling) is becoming an ever more intricate task as the nature of the data offered on the web changes (for example the death of forums in favour of more closed platforms).