Click Chemistry has emerged as a convenient, versatile, and reliable method for biomolecule labeling. One of the most popular click chemistry reactions is the Cu(I)-catalyzed 1,3-dipolar Huisgen cycloaddition of alkynes and azides. This reaction proceeds with great efficiency and selectivity in aqueous media at room temperature to yield a triazole moiety. Another popular click chemistry reaction for bioconjugation and labeling is the strain-promoted copper-free [2+3] cycloaddition. The reaction uses strained cyclooctynes to react with an azide without Cu(I) catalyst at low temperatures with great efficiency. The azide-containing fluorescent dyes can be use for both applications.