Introduction Reviewing the study of personalities of invertebrates offers a series of challenges. First, there is a huge number of invertebrate species, sometimes estimated to represent 98% of the animal species on the planet (Pechenik 2000). Second, invertebrates exhibit a tremendous array of life history strategies, developmental trajectories, modes of reproduction, and physiological bases of behavior, many of which are poorly known. A further challenge arises from the diversity of perspectives and research backgrounds that characterizes invertebrate personality researchers. One of the goals of this review is to determine the degree to which invertebrates, often viewed as animals of limited behavioral repertoires, exhibit personality as defined in the introduction and throughout this volume. We begin with a survey of reports that relate to personality in invertebrates. We categorize these as (1) descriptive reports,(2) physiological/genetic linkages (see Van Oers et al. 2005),(3) ontogenetic studies (sensu West-Eberhard 2003), and (4) ecological/selection studies (Groothuis and Carere 2005; Smith and Blumstein 2008). There will be some bias toward studies of poorly known taxa, even if their evidence is fragmentary. We then evaluate several particularly thorough and influential research programs in depth. In the final section, we provide recommendations for future research directions and attempt to summarize the current state of the field. This review does not evaluate the division of labor (polyethism) in colonies of social insects, or the discrete morphologies (polyphenism) found in many invertebrates. Although we recognize that both of these phenomena may relate to personality (eg, Bergmüller and Taborsky 2010; Bergmüller et al. 2010), we have chosen to focus on subtler forms of personality (ie, those that form continuous rather than discrete distributions). We refer