Plants can sense the presence of neighboring plants to compete with them, to avoid competition with them, or to utilize them as one of survival strategies. The relationships between root parasitic plants and host plants are one of representative examples of plant-plant interactions. Then, what happens between general, non-parasitic plants? It is well known that plants actively interact with other organisms including microbes, but general plant-plant interactions remain poorly understood. In this review, I would like to show that plants detect and respond to neighboring plants by sensing environmental strigolactones.