Ontology learning tools aim to automate the process of building ontologies from various data sources using machine learning and other AI techniques. Most current tools are semi-automatic and require human validation and input. They can learn from text alone using natural language processing, from text combined with existing ontologies, or from structured knowledge bases and ontologies. However, ontology learning remains a challenging task and current tools have limitations such as requiring large amounts of high-quality input data and rules specified by experts.