A variety of different approaches have been used to add digital ink annotations to text-based documents. While the majority of research in this field has focused on annotation support for static documents, a small number of studies have investigated support for documents in which the underlying content is changed. Although the approaches used to annotate static documents have been relatively successful, the annotation of dynamic text documents poses significant challenges which remain largely unsolved. However, it is difficult to clearly identify the successful techniques and the remaining challenges since there has not yet been a comprehensive review of digital ink annotation research. This paper reports the results of a systematic mapping study of existing work, and presents a taxonomy categorizing digital ink annotation research.