This chapter offers an overview of the development of BTP3-Neu5Ac and the functions of sialidase, as revealed through its use. Imaging of sialidase activity in the brain with BTP3-Neu5Ac has demonstrated that high sialidase activity is primarily detected in the white matter. In the hippocampus, a region critical for memory, glutamatergic nerve terminals exhibit particularly high sialidase activity. Based on the sialidase activity atlas, it was found that sialidase plays a crucial role in cognitive function within the hippocampus. Real-time imaging of changes in brain sialidase activity using BTP3-Neu5Ac has shown that sialidase activity at nerve terminals can rapidly increase within seconds in response to neural activity. This rapid fluctuation in sialidase activity contributes to a negative feedback mechanism on excitatory neurotransmission by sialidase. Additionally, studies on the distribution of sialidase activity in the pancreas have shown that the sialidase inhibitor 2,3-dehydro-2-deoxy-N-acetylneuraminic acid (DANA) promotes insulin secretion. Importantly, DANA does not induce insulin secretion during hypoglycemia, suggesting its potential for diabetes treatment with a lower risk of hypoglycemic side effects. Moreover, research into the distribution of sialidase activity in the skin has revealed that the sialidase isozyme NEU2 promotes elastin production. Thus, information on the distribution of sialidase activity in tissues provides valuable insights into the function of sialidase and supports the development of sialidase-targeted drugs.