The boundaries between word classes are often fuzzy. This book looks at the classification of interjections and similar words of other classes. It reviews work done over the past 250 years on several languages, including English, German, French, Italian, Spanish, Latin, Ancient Greek, Albanian, and Welsh. Most chapters discuss interjections in relation to one of the other traditionally recognized parts of speech: nouns, verbs, adjectives, adverbs, adpositions, and conjunctions. A major focus is on the use of relevant terminology e.g. primary and secondary interjections, proper and improper interjections, and interjectives.