We study the problem of maintaining a breadth-rst spanning tree (BFS tree) in partially dynamic distributed networks modeling a sequence of either failures or additions of communication links (but not both). We present deterministic (1 + ε)-approximation algorithms whose amortized time (over some number of link changes) is sublinear in D, the maximum diameter of the network.Our technique also leads to a deterministic (1 + ε)-approximate incremental algorithm for single-source shortest paths (SSSP) in the sequential (usual RAM) model. Prior to our work, the state of the art was the classic exact algorithm of Even and Shiloach [ES81] that is optimal under some assumptions [RZ11, Hen + 15]. Our result is the rst to show that, in the incremental setting, this bound can be beaten in certain cases if some approximation is allowed.Complex networks are among the most ubiquitous models of interconnections between a multiplicity of individual entities, such as computers in a data center, human beings in society, and neurons in the human brain. The connections between these entities are constantly changing; new computers are gradually added to data centers, or humans regularly make new friends. These changes are usually local as they are known only to the entities involved. Despite their locality, they could a ect the network globally; a single link failure could result in several routing path losses or destroy the network connectivity. To maintain its robustness, the network has to quickly respond to changes and repair its infrastructure. The study of such tasks has been the subject of several active areas of research, including dynamic, self-healing, and self-stabilizing networks.One important infrastructure in distributed networks is the breadth-rst spanning (BFS) tree [Lyn96, Pel00]. It can be used, for instance, to approximate the network diameter and to provide a communication backbone for broadcast, routing, and control. In this paper, we study the problem of maintaining a BFS tree from a root node on dynamic distributed networks. Our main interest is repairing a BFS tree as fast as possible after each topology change.Model. We model the communication network by the CONGEST model [Pel00], one of the major models of (locality-sensitive) distributed computation. Consider a synchronous network of processors modeled by an undirected unweighted graph G = (V , E), where nodes model the processors and edges model the bounded-bandwidth links between the processors. We let V and E denote the set of nodes and edges of G, respectively, and let s be a speci ed root node. For any node u and , we denote by d G (u, ) the distance between u and in G. The processors (henceforth, nodes) are assumed to have unique IDs of O(log n) bits and in nite computational power. Each node has limited topological knowledge; in particular, it only knows the IDs of its neighbors and knows no other topological information (such as whether its neighbors are linked by an edge or not). The communication is synchronous and occurs in discrete pulses, calle...