A number of variables have an influence on language learners' success in the learning of a foreign/second language. Some variables, such as motivation, language anxiety or willingness to communicate (WTC), have been researched extensively in the field of second language acquisition, while others, such as boredom, still remain among "the most neglected, insufficiently understood and underexplored emotions" (Pawlak, Zawodniak, & Kruk, 2020c, p. vii) in applied linguistics. This chapter aims to provide an overview of key issues related to the constructs of WTC, motivation, language anxiety and boredom. [For the complete volume, "Investigating Dynamic Relationships among Individual Difference Variables in Learning English as a Foreign Language in a Virtual World. Second Language Learning and Teaching," see ED662008.]