Systems with parametric uncertainty represent an important class of uncertain objects that are characterized by mathematical model containing parameters which are not precisely known, but the values thereof lie within given intervals. This type of uncertainty can arise during the control of real processes, e.g. as a consequence of imprecise measuring or of the influence of certain external conditions. If individual uncertain coefficients (in polynomial, in transfer function etc.) are mutually independent, the uncertainty has a simple interval structure. This article presents several possibilities of interval uncertainty for systems description as well as the tools for robust stability analysis, emphasizing advantages and limitations connected with the use of this simple structures, even for more complex problems.