Two concepts of utmost importance for the analytic philosophy of the twentieth century, “sense-data” and “knowledge by acquaintance”, were introduced by Bertrand Russell under the influence of two idealist philosophers: F. H. Bradley and Alexius Meinong. This paper traces the exact history of their introduction. We shall see that between 1896 and 1898, Russell had a fully-elaborated theory of “sense-data”, which he abandoned after his analytic turn of the summer of 1898. Furthermore, following a subsequent turn of August 1900—-after he became acquainted with the works of Peano and later of Frege—-Russell gradually developed another theory of sense-data. With the collaboration of G. E. Moore, Russell reintroduced the term “sense-data” in 1911. Concomitantly with this move, Russell introduced the epistemological term “knowledge by acquaintance”, which came to designate the grasping of sense-data and universals.