Abstract: This paper reviews the recent advances in urban modeling and urban computing by fusing big data mining algorithms and system science research methods. The complexity of urban systems is first introduced. The key works on the urban sensing and the evolutionary mechanisms as well as empirical studies of the structure of complex urban systems are summarized. In addition, we reveal possible origins of urban scaling laws across cities and spatial scaling laws within cities, and introduce a unified analytical framework to explain and predict complex urban phenomena across various spatial scales.