In ovo electroporation is an epoch-making achievement in the study of developmental biology. With this method, experiments can be carried out in gain and loss of function in desired tissue at any desired stage in chick embryos. Introduction of a tetracycline-regulated gene expression system and a transposon system has further extended the potential of the method, making it possible to obtain long-term expression and to turn on and off a gene of interest. It is now applied to mice, aquatic animals, and even to plants for the study of developmental biology and for other purposes. In this book, the application of electroporation in many embryonic tissues and organs is introduced, with some chapters that deal with gene transfer in adults. Sonoporation, another useful tool, using ultrasonic waves instead of electric currents, for gene transfer to mesenchymal tissues is also introduced.