This book brings together the body of empirical findings and theoretical interpretations of the tip of the tongue (TOT) experience -- when a well-known or familiar word cannot immediately be recalled. Although research has been published on TOTs for over a century, the experience retains its fascination for both cognitive and linguistic researchers. After a review of various research procedures used to study TOTs, the book offers a summary of attempts to manipulate this rare cognitive experience through cue and prime procedures. Various aspects of the inaccessible target word are frequently available -- such as first letter and syllable number -- even in the absence of actual retrieval, and the book explores the implications of these bits of target-word information for mechanisms for word storage and retrieval. It also examines: what characteristics of a word make it potentially more vulnerable to a TOT; why words related to the target word (called "interlopers") often come to mind; the recovery process, when the momentarily-inaccessible word is recovered shortly after the TOT is first experienced; and efforts to evaluate individual differences in the likelihood to experience TOTs.