Stress systems represent an exciting area of research. This chapter introduces the concepts that have played a role in typological discussions of word prominence. It draws a distinction between languages with and without word prosodic representations. The chapter considers how stress manifests itself in languages, keeping the above distinction between phonological and phonetic criteria in mind. It discusses tone systems that resemble stress systems in a distributional sense, making the point that such systems have sometimes been described as stress systems, as well as languages that have neither tone nor stress. The chapter makes the point that stress systems always have a default location, meaning that there are no clear examples of languages which distribute stress entirely unpredictably. It identifies and defines the main parameters that figure in these generalizations on the basis of data from a large number of languages. The chapter also shows that how Optimality Theory has been applied to the problem of deriving default stress distributions.