Aliphatic carbon-halogen (C-X) bonds are prevalent in modern pharmaceuticals and bioactive natural products. Three distinct chemical strategies are known in Nature to generate these structural motifs. The first is via the nucleophilic substitution at a prefunctionalized electrophilic carbon center with a halide anion (X-), known for the S-adenosyl-l-methionine-dependent halogenases. The second is via the electrophilic activation of an alkene or its equivalent by a halenium ion (X+) donor, known for the haloperoxidases and flavin-dependent halogenases. The third is via the direct functionalization of an unactivated aliphatic C-H bond with a halogen radical (X) equivalent, known for the 2-oxo-glutarate and nonheme iron-dependent halogenases. Due to the ubiquitous nature of aliphatic C-H groups in organic molecules, transformations that permit chemo-, regio-, and stereo-selective modification(s) at an unactivated sp3-carbon center have been a long sought-after goal in chemical science. Two nonheme iron-dependent halogenases, WelO5 and AmbO5 involved in the biogenesis of cyanobacterial hapalindole-type alkaloids, have been recently shown able to perform this type of challenging transformation. In this chapter, experimental details for the in vitro reconstitution of WelO5 and AmbO5 enzymatic activities are presented.