Harmful aquatic organisms and pathogens present a major threat to marine ecosystems and shipping has been identified as a significant pathway for introducing species to new environments. The problem has increased with the introduction of steel hulls, allowing ships to use water instead of solid materials as ballast, and in particular over the last few decades as trade and traffic volumes have expanded. The effects of the introduction of non-indigenous species have, in many areas of the world, been devastating. Quantitative data show that the rate of bio-invasions is continuing to increase significantly. As the volumes of seaborne trade continue to increase, the problem may not yet have reached its peak.