Liposomes, that are biodegradable and essentially non-toxic, can encapsulate both hydrophilic and hydrophobic materials, and are utilized as drug carriers for drug delivery systems (DDS). Recent progress in gene technology provides a novel modality of therapy for various diseases with a variety of newlydeveloped cationic liposomes. Delivery of agents to the reticuloendothelial system (RES) is easily achieved since most conventional liposomes are trapped by the RES. For the purpose of delivery of agents to target organs other than RES, long-circulating liposomes have been developed by modifying the liposomal surface. Understanding of the in vivo dynamics of liposome-carried agents is required to evaluate the bioavailability of drugs encapsulated in liposomes. In this review, we focus on the in vivo trafficking of liposomes visualized by positron emission tomography (PET) and discuss the characteristics of liposomes that affect the targeting of drugs in vivo.