In the present work, the onset and subsequent development of slug flow in horizontal pipes is investigated by accurately solving the transient one-dimensional version of the Two-Fluid Model using a finite volume technique. Growth of disturbances that eventually bridge the pipe section is an automatic outcome of the solution of the discretized equations in a high resolution mesh, avoiding the need for the commonly used phenomenological models for the stratified to slug transition. Slug dynamics evolve naturally without the need of empirical correlations for slug parameters. This methodology (named "slug-capturing") was proposed before in the literature and the present work represents a rare confirmation of its applicability in predicting this very complex flow regime. Here, different configurations are analyzed and comparisons are performed against different sets of experimental data. Additionally, statistical analysis of the slug parameters is performed and it is shown through comparisons against experimental measurements that this methodology is able not only to provide mean values of e.g. slug and bubble lengths and their evolution inside the pipe, but also shapes of probability density functions (PDFs), with a good degree of accuracy. Copyright © 2012 by ASME.