This paper traces the time series tradition in the study of market structure, focusing on how recent studies on entry and the size distribution of firms have modified thinking in this area. It begins with Robert Gibrat's 1931 work, which introduced the "Law of Proportional Effect," suggesting that the expected growth rate of a firm is proportional to its current size. Gibrat's model, based on the assumption that firm growth is stochastic and independent of size, was later challenged by empirical studies showing that growth rates are not always independent of firm size. The paper discusses the development of the literature, including the introduction of game-theoretic models and the shift towards more realistic models that incorporate stochastic elements and industry-specific characteristics. It also examines the role of firm size and growth in different industries, highlighting the importance of industry-specific factors such as scale economies, advertising, and R&D. The paper concludes by noting the importance of understanding the size distribution of firms in different industries and the need for further research into the factors that influence this distribution.This paper traces the time series tradition in the study of market structure, focusing on how recent studies on entry and the size distribution of firms have modified thinking in this area. It begins with Robert Gibrat's 1931 work, which introduced the "Law of Proportional Effect," suggesting that the expected growth rate of a firm is proportional to its current size. Gibrat's model, based on the assumption that firm growth is stochastic and independent of size, was later challenged by empirical studies showing that growth rates are not always independent of firm size. The paper discusses the development of the literature, including the introduction of game-theoretic models and the shift towards more realistic models that incorporate stochastic elements and industry-specific characteristics. It also examines the role of firm size and growth in different industries, highlighting the importance of industry-specific factors such as scale economies, advertising, and R&D. The paper concludes by noting the importance of understanding the size distribution of firms in different industries and the need for further research into the factors that influence this distribution.