Quasi-categories or (infinity,1)-categories are a possible model for weak infinity-categories. This notion is an important building block in Jacob Lurie's treatise ``Higher Topos Theory''. Quasi-categories were introduced by Boardman and Vogt under the name ``weak Kan complexes'' and then studied by Andr\'e Joyal. I will start my talk by recalling simplicial sets and closed model categories. Then, I will introduce quasi-categories and give a few examples. I will discuss relations between quasi-categories, topological categories and simplicial categories. Finally, I will show that quasi-categories form a closed model category.