Larval ascariasis is much less well understood than adult worm infection. Part of the explanation for this is the difficulty in assessing the impact of larval infections in human subjects. In contrast, adult worms can be counted after post-chemotherapeutic expulsion or, additionally, eggs voided in host feces can be quantified as an indirect measure of infection intensity. Furthermore, respiratory and hepatic disease, which may be associated with transient passage of larvae through these organs, can be either non-specific or cryptic, making Ascaris-specific diagnosis through clinical signs of infection challenging. The influence of early infection on parasite establishment and host immunity is recognized as important but it remains challenging to unravel the complexities of the relationship. Both pigs as natural hosts of Ascaris suum and a range of abnormal hosts have been utilized to investigate larval migration. Few if any of these model systems have enabled clarification of the basis of variation in susceptibility and resistance to larval migration and accumulation in the liver and lungs. In this chapter, we document the literature on larval ascariasis with a particular emphasis on studies utilizing mice because mice are now developing as useful model systems for the study of early Ascaris infection, the migratory path and its duration being similar to that of pigs. We describe a mouse model of early Ascaris infection that mimics the extremes of the host phenotype displayed in the aggregated distribution of adult worms in human and pig populations, outline the existing evidence for the role of the liver in susceptibility to larval ascariasis and discuss future directions for the use of this model system.