The Unified Modeling Language (UML) is a visual modeling language for documenting and specifying software. It is gaining popularity as a language for a variety of purposes. It was designed as a result of a unifying activity in the last decade. Since this general purpose language cannot suit all possible needs, it has built-in mechanisms for providing extensibility for specific purposes. One such mechanism is the notion of stereotype, which is a means of branding the existing model element with a new semantics. Such extended elements can then act as new model elements as if they were standard model elements. This notion is only one of the possible ways of customizations of the language. The other, more powerful technique is metamodeling, which enables to change UML by directly changing its specification. The thesis investigates the notion of stereotype in UML both from theoretical and practical perspectives. It examines the notion of stereotype as it originally appeared in object-oriented software development as a means of branding objects according to their secondary classification in the system. The initial intent behind stereotypes is compared with the view of stereotypes in UML and similar languages, which later on provides a basis for an understanding of a stereotype in the thesis. The thesis elaborates on a classification of stereotypes from the perspective of their usage. The classification categorizes different usages of stereotypes in different situations. Based on the classification, one such usage is evaluated in an empirical way. The evaluation is done in the form of an experiment on how the stereotypes influence the understanding of UML models. An example of a customization of UML for a conceptual database model is presented. It is a basis for a study on the expressiveness of stereotypes in the context of persistency modeling in objectoriented software. Two ways of the introduction of the stereotypes into the software development process (dependent and independent of UML tools) are outlined.The thesis contains also a presentation of how the knowledge expressed as ontology can be imported into domain models expressed in UML. This research can be seen as a further study on the customization of UML towards usage of ontology-based knowledge.