  Much research effort has been devoted to developing methods for
reconstructing the links of a network from dynamics of its nodes. Many current
methods require the measurements of the dynamics of all the nodes be known. In
real-world problems, it is common that either some nodes of a network of
interest are unknown or the measurements of some nodes are unavailable. These
nodes, either unknown or whose measurements are unavailable, are called hidden
nodes. In this paper, we derive analytical results that explain the effects of
hidden nodes on the reconstruction of bidirectional networks. These theoretical
results and their implications are verified by numerical studies.
