This chapter reviews the research that has led to the first successes in genetic transformation of arthropod-borne bacteria belonging to the order Rickettsiales in the Alphaproteobacteria subdivision. Seminal results provided the very first evidence that direct genetic manipulation of rickettsiae was achievable, and that the bacteria were able to maintain and express foreign genetic sequences inserted via allelic exchange/homologous recombination under control of rickettsial or Escherichia coli promoters. This work set the stage for all subsequent research efforts aspiring to manipulate and analyze rickettsiae in a manner that is nearly commonplace in extracellular bacteria. The Himar1 transposase allele A7 has been successfully used for mutagenesis of A.phagocytophilum and fever groups of rickettsial tick symbionts have been spotted using fluorescent markers and antibiotic resistance. Selection of mutants using growth inhibitors is an indispensible strategy to recover mutants from the background of nontransformed bacteria. The most common strategy is incorporation of an antibiotic resistance gene in the transformation cassette. When working with human or animal pathogens, this is a sensitive issue, as introduction of resistance to antibiotics used to treat disease induced by the pathogen to be transformed is not encouraged. This poses a dilemma, as the most effective selection is likely achieved by using the clinically most effective antibiotics. The authors suggests that shuttle vectors would be useful for testing the function of genes that are naturally defective in certain Rickettsia spp., in complementation assays, overexpression of native or foreign genes, testing gene regulation, and other applications carried out in E.coli.