Trace metals (TMs) have a central role in the functioning of aquatic and terrestrial ecosystems. Both essential and toxic trace metals are involved in different chemical, biological, and physical processes that determine their concentrations, speciation, reactivity, and impact on the biota. Despite the recognized hazards associated with the uncontrolled release of metals in the environment and the measures to reduce their input, metals remain an important threat to terrestrial and aquatic ecosystems. Several metals were included on the list of priority substances of environmental concern, whereas technology-critical elements (TCEs), such as rare-earth elements (REEs) and platinum group metals (PGMs) emerge as contaminants. Important advances were achieved in the past decades with respect to the fundamental understanding of trace metal behavior in the aquatic and terrestrial ecosystems, as well as the role of different modifying factors. On this basis, a key concept such as TMs speciation and bioavailability were developed and revealed the utmost role of the interrelated dynamic biological, chemical, and physical processes in TMs cycling and regulation. Nevertheless, there are some persistent gaps in the knowledge challenging the environmental toxicology of TMs, especially for the conditions that are the most relevant to the natural and impacted environment, such as the presence of multiple stressors and metal mixtures, contaminants of emerging concerns etc. This Special Issue provides some innovative insights into trace metals in the aquatic and terrestrial environments addressing several of the current gaps in contemporary environmental toxicology.