Liquid chromatography mass spectrometry (LC-MS) has evolved as a powerful tool in protein and peptide identification and quantitation. It has the capability to monitor thousands of proteins and peptides simultaneously and therefore is a critical component in the study of underlying biological mechanisms and the discovery of novel therapeutic targets. This dissertation is devoted to the development and application of novel LC-MS based techniques to characterize proteomics and peptidomics qualitatively and quantitatively by incorporating various sample preparation strategies, novel MS methods and isobaric labeling technology. A portion of this dissertation describes the design and evaluation of several dimethylated amino acid tags and their potentials to be used as isobaric labeling reagents for protein and peptide quantitation. Their utilities are further demonstrated through their application in the study of global proteomic changes in models of restenosis. Although isobaric tags are originally designed for higher-energy collisional dissociation (HCD), we expand their use by enabling them to be coupled with electron-transfer/higher-energy collision dissociation (EThcD) fragmentation. The benefits of using such strategy in quantitative proteomics, especially phosphorylation studies, are recounted in this dissertation. Quantitative strategies presented in this dissertation are complemented by advances in MS-based protein and endogenous peptide sequencing methodologies. We established a workflow to sequence intact glycopeptides, enabling detailed investigation of complex protein glycosylation and its microheterogeneity. The application of this improved workflow revealed a hyperglycosylation trend from day 0 to day 7 post angioplasty along the progression of restenosis. Similarly, by employing a further modified strategy, this dissertation documented the discovery of O-linked glycosylation on endogenous signaling peptides, specifically insulin and its related peptide hormones. The significance of such discovery can be critical toward diabetes research and treatment. Meanwhile, this dissertation also describes the strategy to characterize large neuropeptides in a species with no genome information, which can be greatly useful for future neuropeptide research. Furthermore, we employ ion mobility MS (IM-MS) to study peptide structures and conformations, leading to discovery of D-amino acid containing peptides based on subtle changes in gas-phase conformations despite identical mass-to-charge ratio. The IM-MS strategy enables the study of this unique isobaric post-translational modification (PTM) in neuropeptides occurring via amino acid isomerization. Overall, this dissertation research not only improves upon isobaric labeling technique by exploring novel chemical tags and new fragmentation methods, but also presents a useful platform to enable in-depth investigation on complex PTMs and subtle conformational differences. Collectively, we expect that the technology advancements presented in this work will enable broad applications in various biological systems and lead to improved understanding of underlying molecular mechanisms.