Network biology is conceptualized as an interdisciplinary field, lying at the intersection among graph theory, statistical mechanics and biology. Great efforts have been made to promote the concept of network biology and its various applications in life science. In this review, we focus on the modules that are functional entities and building blocks of a complex network. We first introduce the basic concepts and hot spots of network biology, including several network models, general design principles of complex networks in the module point of view, module discovery approaches and the conservation and evolvability of modules. We then present several cases in which the important mechanisms underlying the cellular behavior are revealed in the framework of network, especially, in the concept of module.