Parasitic plants use a special organ, the haustorium, to attach to and penetrate host tissues, forming phloem and/or xylem fusion with the host vascular systems. Across this haustorium–host interface, not only water and nutrients are extracted from the host by the parasitic plant, but also secondary metabolites, messenger RNAs, noncoding RNAs, proteins, and systemic signals are transported between the parasite and host and even among different hosts connected by a parasite. Furthermore, mycorrhizal fungi can form common mycelial networks (CMNs) that simultaneously interconnect multiple plants. Increasing lines of evidence suggest that CMNs can function as conduits, transferring stress-related systemic signals between plants. Between-plant signaling mediated by haustoria and CMNs likely has a profound impact on plant interactions with other organisms and adaptation to environmental factors. Here, we summarize the findings regarding between-plant transfer of biomolecules and systemic signals and the current understanding of the physiological and ecological implications of between-plant signaling.