The myriads of protein post-translational modifications (PTMs) are major factors causing an exponential increase of complexity in the human proteome. Despite that the PTMs play key functional roles in cellular processes, the kinds of modifications, sites occupied, dynamics and interplay are far from understood. Glycosylation is one of the most abundant posttranslational modifications and is involved in many key biological events. My research is focused on the development of chemistry-based tools for chemical biology studies of the human glycoproteome. Additionally we aim to contribute to the development of therapeutic and diagnostic applications specifically centered on glycosylation events in cancer and infection biology. In this Beilstein Talk I will present our most recent research work on the development of new glycoproteomic detection tools to study new tyrosine O-glycosylation PTMs and mucin type O-glycosylation. The synthesis and application of glycopeptide libraries to study the roles of fucose and sialic acid modifications in protein binding events will further be presented.