The concept of "click" chemistry, introduced by Sharpless and coworkers a couple of years ago, promotes the use of efficient, selective and versatile chemical reactions in synthetic chemistry. For instance, the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) is regarded as a prime example of "click" chemistry. This reaction is regioselective, chemoselective and moreover can be performed in aqueous medium at room or physiological temperature. Thus, CuAAC became lately a very popular ligation tool in biological and medical sciences. Several hundred of articles exploring the synthetic possibilities of CuAAC in biosciences have been published within the last four years. The aim of the present review is to give an overall--non exhaustive--picture of this emerging field of research. The advantages and versatility of CuAAC in scientific disciplines as diverse as drug discovery, biochemistry, bioconjugates synthesis, drug-delivery, gene therapy, bioseparation or diagnostics are presented and discussed in detail.