Risk assessment is a tool that has many applications in marine biosecurity. Its application to aquaculture has only recently moved from the protective standpoint of animal health (i.e. the World Organisation for Animal Health, OIE) to examining introduced species risks. Risks from aquaculture include use of non-native species as target stocks in aquaculture; the potential for introductions of hitchhiker (associate) species when importing new stocks; the use of non-native live, fresh or frozen feed stocks and the movement of aquaculture equipment. In contrast, the risks to aquaculture from marine bioinvasions from other sources (including other aquaculture operators) include pathogens, parasites, biofouling and harmful algal blooms. Herein, we present two types of risk assessment (non-probabilistic decision-trees and a qualitative/semi-quantitative organism impact assessment) that are currently used in the marine biosecurity system in New Zealand and Chile, but are readily applicable to other introduced species risk scenarios. These methods do not rely on quantitative risk assessment methods because sufficient quantitative data are often lacking for introduced marine species work. However, quantitative data can be used within the assessments to identify likelihoods or consequence.