It is desirable to automate all the components in the framework for ontology learning by developing techniques. However, this remains infeasible and some components are carried out manually. For example, Zhou et al. first crafted the top-level ontology manually based on existing domain resources, then learned ontological concepts and relations automatically, and finally mapped learning results to the top-level ontology by hand. Riloff and Shepherd initiated the learning process with a set of manually selected seed concepts, which were then bootstrapped gradually. We summarize and analyze ontology learning approaches in the next section.