This work describes the technique of microinjection used to transfer biological materials - RNA, proteins, oligonucleotides and especially cloned genes - into cultured somatic cells or embryos. Transgenic cells or animals - mice, rabbits or insects - allow the study of a range of cellular processes, such as gene expression, signal transduction or the cytoskeleton. More than 30 protocols on microinjection experiments and transgenesis studies developed in laboratories are described in this manual. Special topics are the generation of transgenic mice using YAC, ligand-dependent site-specific recombination in embryonic stem cells, morula aggregation to generate germline chimeras as well as the analysis of injected cells.