The technique of needle microinjection has become a prominent experimental approach in biological research. Cellular organelles, DNA and RNA, enzymes, structural proteins, metabolites, ions and antibodies are just some of the molecular and cellular elements that have been transposed from test tubes into living cells by needle injection. This technique is broadly used as a valuable tool for the study of many different cell responses in a variety of systems. This text presents information on the technique of needle microinjection's principles, the required equipment, preparation of receiving cells and injected material and the analysis of the results. It provides detailed protocols for a wide range of important applications and different cellular systems from the fields of cell cycle regulation, signal transduction, study of transcriptional regulations, cytoskeletal functions, secretion and intracellular transport.