Porous carbons are an attractive material for electrochemical energy storage technologies due to their light weight, low cost, good electrical conductivity, and easily obtainable porosity. Lithiumsulfur (Li-S) batteries in particular are an attractive technology due to the light weight and low cost of sulfur, and a high theoretical specific capacity (1672 mA h g-1) and specific energy (2567 W h kg-1) compared to current lithium-ion batteries. Porous carbons help to disperse the insulating sulfur at a nanoscale generating close electrical contact and making it electrochemically active. Moreover, the pores can act as small reservoirs for the soluble reaction intermediates preventing their egress from the electrode, a phenomenon responsible for capacity decay over cycling. However, dependence of the overall Li-S battery performance on carbon's porosity characteristics (surface area, pore volume, electrical conductivity) is still not completely understood. In this dissertation, by utilizing the highly tunable Hierarchical Porous Carbons (HPCs) developed in our group as a model system, that dependence was studied in a systematic way. Carbons with widely ranging porosity were synthesized and trends correlating porosity with sulfur utilization and cyclic performance were found. These correlations can be used not only as a useful guide for preparing novel porous materials for this application but also hint on what other factors besides porosity, for example electrical conductivity, may be limiting the performance and hence need more attention. To further study the effect of electrical conductivity of carbons, catalytically graphitized hierarchical porous carbons were synthesized and tested in lithium/sulfur batteries and a remarkable improvement in long term cycle life was observed. Additionally, a novel method to synthesize a series of nitrogen doped mesoporous carbons with very high and tunable nitrogen contents (up to 21 at. %) is presented. These carbons showed promising performance as supercapacitor electrodes and can be potentially used for other application as well such as metal-free oxygen reduction reaction (ORR) catalysts and as lithiumion battery anodes.