Although many ecologists are aware of the presence and importance of natural enemies in the soil that protect the plants from herbivores, the existence and nature of tritrophic interactions are poorly understood. So far, attention has focused on how plants protect their above-ground parts against herbivorous arthropods, either directly or indirectly (i.e. by getting help from the herbivore's enemies). We provided the first evidence that indirect plant defences also operate underground. Chemicals released from the roots of Thuja occidentalis and Taxus baccata when attacked by weevil larvae Otiorhynchus sulcatus attracted entomopathogenic nematodes Heterorhabditis megidis. Recently a first semiochemical involved was identified in maize plants damaged by larvae of Diabrotica virgifera virgifera. It is expected that tritrophic interactions in soil are not only limited to entomopathogenic nematodes as natural enemies. A new discovery of a tritrophic relation between an entomopathogenic fungus in relation to plant and pest insect is such a new example. Next to trophic relations in the soil there are also cross effects between below- and aboveground. We will discuss options of tritrophic interactions in soil as well as below- and aboveground relations that may exist and how these interactions may provide us with new tools in plant production systems.