The first genetic manipulations applied to Toxoplasma were performed by using chemical mutagenesis. The reverse genetics approach, which introduces foreign DNA into parasites, was achieved using electroporation. Initially the transient transfection of plasmid DNA containing reporter genes flanked by T. gondii 5’ and 3’ flanking sequences allowed the expression of reporter genes used for the characterization of the elements controlling transcription. This methodology was rapidly utilized to identify and validate several selectable marker genes, which then opened an avenue for stable transformation and the development of invaluable panoply of tools associated with DNA transfection. A wide range of positive and negative selectable markers has been tailored for homologous recombination leading to allelic replacement and gene knockouts.