Before discussing multipartite entanglement, it is necessary to start with the simplest case of bipartite entanglement, involving only two parties. This chapter begins with the exploration of bipartite entanglement by revisiting the famous paradox of “spooky action at a distance,” as described by Einstein, Podolsky, and Rosen. Next, various methods for detecting entanglement are introduced. Subsequently, we present the definition of entanglement measures and entanglement monotones, accompanied by the necessary requirements that those measures or monotones must satisfy. To conclude, several existing bipartite entanglement measures are introduced.