This chapter is structured as follows. In Sect. 2.1, we compile a systematic literature review of (English) EL with Wikidata definitions, tasks, approaches, datasets, types, domains, applications, and challenges. Based on the results, we identify the research gap of MathEL (mathematics as missing EL domain, and formula as missing EL entity type). In Sect. 2.2, we review mathematics in Wikidata. This includes how to retrieve and employ the knowledge stored in Wikidata items that represent mathematical concepts.