This review presents principles of glycosylation, describes the relevant 
glycosylation pathways and their related disorders, and highlights some of the 
neurological aspects and issues that continue to challenge researchers. More 
than 100 rare human genetic disorders that result from deficiencies in the 
different glycosylation pathways are known today. Most of these disorders impact 
the central and/or peripheral nervous systems. Patients typically have 
developmental delays/intellectual disabilities, hypotonia, seizures, neuropathy, 
and metabolic abnormalities in multiple organ systems. Among these disorders 
there is great clinical diversity because all cell types differentially 
glycosylate proteins and lipids. The patients have hundreds of misglycosylated 
products, which afflict a myriad of processes, including cell signaling, 
cell-cell interaction, and cell migration. This vast complexity in glycan 
composition and function, along with the limited availability of analytic tools, 
has impeded the identification of key glycosylated molecules that cause 
pathologies. To date, few critical target proteins have been pinpointed.