Usability has been increasingly recognized as an important factor in the acceptance of systems by end users. Usability requirements can be considered to be requirements that capture the usability goals and associated measures for a system under development. In order to ensure usable systems we must ensure identification of appropriate requirements regarding these critical aspects of systems. There is a basic need for systematic approaches to reason, model and analyze usability from the early stages of the software development. Furthermore, it is necessary to develop a usable ontology or classification of measurable aspects of usability that can be used to aid in the specification of usability requirements. These ontologies should be represented in a way that facilitates their use as guidelines for the requirements elicitation process. We build on review of literature in the area of human-computer interaction and of usability engineering in developing a catalog of aspects of usability that can be considered during requirements gathering. This catalogue is used to guide the requirements engineer through alternatives for achieving usability. The approach is based on the use of the i* framework, having usability modeled as a special type of goal.