We study the evolution of the influence of journals over the period 1970-2017. In the early 1970's, a number of journals had similar influence, but by 1995, the `Top 5' journals ?? QJE, AER, RES, Econometrica, and JPE ?? had acquired a major lead. This dominance has remained more or less unchanged since 1995. To place these developments in a broader context, we also study trends in sociology. The trends there have gone the other way ?? the field journals rose in influence, relative to the Top General journals. A model of journals as platforms is developed to understand these trends across time and across disciplines.