  Machine learning techniques are now integral to the advancement of
intelligent urban services, playing a crucial role in elevating the efficiency,
sustainability, and livability of urban environments. The recent emergence of
foundation models such as ChatGPT marks a revolutionary shift in the fields of
machine learning and artificial intelligence. Their unparalleled capabilities
in contextual understanding, problem solving, and adaptability across a wide
range of tasks suggest that integrating these models into urban domains could
have a transformative impact on the development of smart cities. Despite
growing interest in Urban Foundation Models~(UFMs), this burgeoning field faces
challenges such as a lack of clear definitions, systematic reviews, and
universalizable solutions. To this end, this paper first introduces the concept
of UFM and discusses the unique challenges involved in building them. We then
propose a data-centric taxonomy that categorizes current UFM-related works,
based on urban data modalities and types. Furthermore, to foster advancement in
this field, we present a promising framework aimed at the prospective
realization of UFMs, designed to overcome the identified challenges.
Additionally, we explore the application landscape of UFMs, detailing their
potential impact in various urban contexts. Relevant papers and open-source
resources have been collated and are continuously updated at
https://github.com/usail-hkust/Awesome-Urban-Foundation-Models.
