This paper reviews the results of 13 empirical studies investigating associations between L1 and L2 reading or writing skills, that were published after the reviews of Alderson (1984) and Cummins (1991a). Special attention is given to two large studies, in which the author was involved, that use covariance structure analysis, dealing with measurement error more adequately than previous studies. The paper then critically examines the threshold hypotheses of Cummins (1979) and Alderson (1984) by distinguishing language-dependent knowledge and skills from general cognitive abilities. The paper ends with the outline of a language ability theory, containing the constructs of basic language cognition (BLC) and higher language cognition (HLC), as the basis for a program of research into the commonalities and differences among L1-users 
(and L2 users) in performing reading, writing, listening and speaking tasks.