Research and Development (R&D) service firms make significant contributions to innovation in other businesses. The extant literature considers these firms a homogenous sub-group of Knowledge Intensive Business Service firms (KIBS). The objective of this study is to investigate how R&D service firms innovate and the variety of innovation practices within these firms. Employing data from semi-structured interviews with senior managers from 32 UK-based R&D service firms, we suggest that there are two different modes of innovation: in the first mode, R&D service firms innovate similarly to KIBS whose innovation is ad-hoc in nature and driven by customers’ requests; another group of R&D service firms innovate like New Technology-based Firms (NTBFs) relying more on structured in-house R&D activities.