In this chapter the author gets involved with transaction platforms or multi-sided platforms that facilitate transactions between different types of actors. Transaction platforms are usually platform intermediaries. All platforms are digital: they capture, transmit, and monetize data over the internet and other networks, but they may as well have some physical elements included in the product offering. The author provides a platform innovation taxonomy that is a conceptualization and formalization of elements, relationships, vocabulary, and semantics of different dimensions of transaction platform innovations.