In many ecosystems, apex consumers (top predators) play acrucial role in food web dynamics. Their top-down impactsare known to spread downwards through the food webs ina cascading fashion. Trophic cascades manifest when toppredators are excluded from an ecosystem or introduced intoa system lacking them. Two such case studies are describedhere. Trophic cascades are generally more pronounced in theaquatic than in the terrestrial ecosystems, which in turn isrelated to the differences in the average body sizes of herbivoresrelative to the plant resources they feed upon andin the structure of their food webs. Scientists express concernthat human activities have been leading to planet-widetrophic downgrading (loss of apex consumers) which couldaffect vital ecological processes of the biosphere.