Obligate predatory bacteria, i.e. bacteria requiring a Gram negative prey cell in order to complete their cell cycle, belong to the polyphyletic group referred to as the Bdellovibrio And Like Organisms (BALO). Predatory interactions between bacteria are complex, yet their dynamics and impact on bacterial communities in the environment are becoming better understood. BALO have unique life cycles: they grow epibiotically with the predator remaining attached to the prey's envelope, dividing in a binary manner or periplasmically, i.e. by penetrating the prey's periplasm to generate a number of progeny cells. The periplasmic life cycle includes unique gene and protein patterns and unique signaling features. These ecological and cellular features, along with applications of the BALO in the medical, agricultural and environmental fields are surveyed.