This book argues (a) that there is no principled way to distinguish inflection and derivation and (b) that this fatally undermines conventional approaches to morphology. Conceptual shortcomings in the relation between derivational and lexically-derived word forms, Andrew Spencer suggests, call into question the foundation of the inferential-derivational approach. Prototypical instances of inflection and derivation are separated by a host of intermediate types of lexical relatedness, some discussed in the literature, others ignored. Far from finding these an embarrassment Professor Spencer deploys the wealth of types of relatedness in a variety of languages (including Slavic, Uralic, Australian, Germanic, and Romance) to develop an enriched and morphologically-informed model of the lexical entry. He then uses this to build the foundations for a model of lexical relatedness that is consistent with paradigm-based models. Lexical Relatedness is a profound and stimulating book. It will interest all morphologists, lexicographers, and theoretical linguists more generally.