  Recent years have witnessed the proliferation of fake news, propaganda,
misinformation, and disinformation online. While initially this was mostly
about textual content, over time images and videos gained popularity, as they
are much easier to consume, attract much more attention, and spread further
than simple text. As a result, researchers started targeting different
modalities and combinations thereof. As different modalities are studied in
different research communities, with insufficient interaction, here we offer a
survey that explores the state-of-the-art on multimodal disinformation
detection covering various combinations of modalities: text, images, audio,
video, network structure, and temporal information. Moreover, while some
studies focused on factuality, others investigated how harmful the content is.
While these two components in the definition of disinformation -- (i)
factuality and (ii) harmfulness, are equally important, they are typically
studied in isolation. Thus, we argue for the need to tackle disinformation
detection by taking into account multiple modalities as well as both factuality
and harmfulness, in the same framework. Finally, we discuss current challenges
and future research directions.
