The tip-of-the-tongue (TOT) experience is a universal experience in which a speaker cannot fully produce a word that he or she believes will eventually be recalled and could easily be recognized. The purpose of the current set of experiments is to determine how different variables affect the rate of TOTs. Specifically, a series of three experiments investigates the roles of word concreteness and word frequency on TOT rates. A new finding, the concreteness effect on TOT rates, emerged and was replicated across all three experiments. This never-before investigated concreteness effect is discussed in terms of a general two-stage model of language production. The findings best fit the strength of connections hypothesis of TOTs, which explains TOTs as a failure of the connection from a word's semantic representation to its phonological representation. Implications for theories concerning the mental representation and access of concrete words are discussed. Additionally, a discussion concerning the proper measurement of TOT effects is included.