Introduction

In this first section a brief introduction to hard photoproduction is presented. Then the general characteristics of the photoproduction events which give rise to rapidity gaps in the final state are described and diffraction is defined in this context. The events may be classified into two groups, those which give rise to a central rapidity gap, and those which give rise to a forward rapidity gap. The results which have been obtained by the ZEUS Collaboration from the study of these two classes of events are presented and discussed in the following two sections. These are published results [1,2], and the reader is referred to the publications for detailed accounts of the event selection, the Monte Carlo event generation and the corrections for detector effects. Some concluding remarks and an outlook are provided in the final section.