Porous metal-organic frameworks (MOFs) represent a new type of functional materials and have been found to exhibit great potential in various applications such as catalysis, magnetism, gas storage/separation etc. This dissertation details the investigation of porous MOFs for gas adsorption applications, including hydrogen storage, methane storage, and selective gas adsorption. The first section evaluates porous MOFs as promising candidates for hydrogen storage application. It discusses various strategies to improve hydrogen uptakes in porous MOFs, which includes mimicking hemoglobin to create entatic metal centers in PCN-9 resulting in a high hydrogen heat of adsorption of 10.1 kJ/mol, functionalizing the organic ligand with fused aromatic rings to achieve high hydrogen adsorption capacity of 2.7 wt% in PCN-14 at 77 K and 1 bar, and utilizing catenation to generate PCN-6 with a hydrogen uptake of 9.5 wt% (absolute, at 77 K and 50 bar) as well as a surface area of 3800 m2/g in. The second section discusses methane storage applications of porous MOFs. Constructed from a pre-designed ligand, the porous MOF, PCN-14 exhibits the highest methane uptake capacity among currently reported materials with a value of 230 v/v (absolute, at ambient temperature and 35 bar), which is 28% higher than the US DOE target (180 v/v) for methane storage. The third section addresses microporous MOFs as molecular sieves for selective gas adsorption application. Increasing the bulkiness of the struts and introducing coordinatively linked interpenetration restrict the pore sizes of PCN-13 and PCN-17 respectively to scelectively adsorb oxygen and hydrogen over nitrogen and carbon monoxide. Based on some amphiphilic ligands, a series of mesh-adjustable molecular sieves, whose pore sizes can be continuously tuned from 2.9 to 5.0 angstrom, have been designed for various gas separation applications.