Anaplasma phagocytophilum is the etiologic agent of granulocytic anaplasmosis, a tick-borne, zoonotic, emerging infectious disease. A. phagocytophilum is an obligate intracellular pathogen that primarily resides within membrane-bound, cytoplasmic vacuoles of host neutrophils. Closely related to Ehrlichial and Rickettsial organisms, A. phagocytophilum is a small, fragile, Gram-negative bacterium that presents unique challenges for culture, isolation, enumeration, and labeling. This chapter delineates pathogen-specific considerations for culture and labeling of this organism for subsequent use in assays to examine mechanisms of host cell–pathogen interactions.