Graphs are the mathematical models for networks. Expander graphs are well-connected yet sparse graphs. The expansion property of a regular or bi-regular graph is governed by the second largest eigenvalue of its adjacency matrix. Optimal expanders are called Ramanujan graphs. We will introduce the notion of primes for graphs and define the Ihara-Zeta function and the Riemann Hypothesis in the context of graphs. Graphs satisfying the Riemann Hypothesis are Ramanujan. We will use methods from the representation theory of p-adic groups to construct infinite families of (regular and bi-regular) Ramanujan graphs.