Valley networks are frequently cited as the best evidence that liquid water once existed on Mars. It now appears that they were active during a specific period of time in Martian history that occurred during a "climatic optimum" when climatic conditions supported rainfall, runoff, evaporation, and all the other elements that make up a hydrologic cycle. To better understand how and when valley networks formed, we have tested a variety of proposed age-dating techniques. Each of these techniques has their own merits and can provide important clues about when the valley networks were active and how they even transported sediment. We have also characterized the morphology of valley networks and found that they were influenced by evaporation from the northern plains, supporting possible evidence for a global ocean. Currently, we are examining valley networks in other areas of Mars with the goal of understanding potential global and regional climatic differences.