It is the author’s hope that this document will help provide the novice with both an understanding of what dynamic aperture is and some insight as to why it is limited. In addition, we hope that this paper will provide information as to the analytical and numerical methods available for the study of dynamic aperture. Finally, we wish to present some interpretations and applications of these concepts and tools by means of examples. Those factors having an impact on the dynamic aperture of circular machines and storage rings will be discussed; applications to two types of machine of current popular interest (namely, large hadron colliders and synchrotron radiation electron storage rings) will be made in order to illustrate the ideas that are presented.