Summary (With Shinjiro Yamaguchi) Strigolactones (SLs) are multifunctional hormones that contribute to the control of shoot branching, and serve as signals between plants and arbuscular mycorrhizal fungi and between host plants and parasitic Striga plants. They are produced in minute quantities and so studying strigolactones has been difficult. Strigolactone research has benefited from the identification of pea, Arabidopsis and rice mutants affected in SL synthesis or signaling. Because they contribute to beneficial symbiotic interactions with mycorrhiza, as well as detrimental interactions with parasitic plants, understanding how SLs are produced and act is a high priority particularly in tropical regions where Striga parasites are endemic. Progress in the past decade has been rapid, but we have much still to learn about this fascinating family of hormones.