Long-standing major questions in auxin biology are now being answered through the latest discoveries and characterizations of auxin receptors and transporters. An F-box protein TIR1 and its close homologs are emerging as potent auxin receptors, which directly modulate the degradation of transcriptional repressors for auxin-responsive genes. The membrane proteins for polar auxin transport, intuited by Darwin almost 130 years ago, have been characterized over the past decade and implicated in diverse aspects of auxin-mediated plant development. This growth regulator is now considered to be a plant equivalent of morphogen because of how crucial the formation of its transporter-associated concentration gradient is to the patterning processes of plants. Such long-distance auxin-signaling from the source to the target cell via transporters has helped advance our understanding of plant development as a holistic system. Here, we summarize recent achievements in the study of molecular and long-distance signaling mechanisms for auxin, and discuss their biological meaning.