On the eve of 20th century, three papers launched the modern Network Science by bringing it to the attention of a wider community of physicists, computer scientists and applied mathematicians. The papers – by Watts and Strogatz , Barabasi and Albert , and Google founders Brin and Page  – introduced “small world networks”, “preferential attachment,” and “PageRank” into the vernacular of network scientists. They showed that simple models could reproduce much of the complexity observed in network structure and that the structure of networks was linked to their function. As we mark the 20th anniversary of the publication of these seminal works, it is time to reflect on the state of Network Science and where the field is headed. What have we learned about networks over the past two decades? How does network structure affect its function? How do we represent networks, predict and control their behavior? How do networks grow and change? What are the limits of our understanding, and finally, what are the important open problems in network science?