I develop and test theoretical hypotheses for how employees' authenticity at work influences their motivational, relational, and effectiveness outcomes. These hypotheses are grounded in the idea that when individuals feel they display their true selves at work, they can more fully employ their physical, cognitive and emotional energies in their work roles, which in turn leads to higher levels of employee effectiveness (e.g., task performance, perceived value to the organization, and promotability). In addition to this personal motivational process, individuals who are more authentic also develop high-quality relationships with their coworkers, thereby receiving more instrumental support and minimizing the antagonistic encounters they have with their colleagues. Both types of coworker interactions should, in turn, also influence the focal individual's effectiveness at work. Finally, I hypothesize that the relationships between authenticity and these relational and effectiveness outcomes are moderated by certain personality traits, such that when an individual is highly narcissistic, has very low self-esteem, or has strongly held values or beliefs generally perceived to be negative or deviant, the relationships change: authenticity's positive influence on coworker instrumental support becomes less positive, and authenticity's negative influence on coworker incivility becomes less negative. These moderation effects are expected for employee effectiveness as well. The sample used to test these hypotheses consisted of 102 employees and their 16 supervisors from two private companies headquartered in the Southwest United States. Authenticity was found to be positively associated with employee engagement, coworker instrumental support, and employee effectiveness, and negatively associated with coworker incivility. Once other factors were controlled for, significant relationships remained with employee engagement and coworker support. Contrary to expectations, neither engagement nor coworker interactions mediated the authenticity-employee effectiveness relationship. A dark side of authenticity was found for two of the three personality traits: self-esteem moderated the relationship between authenticity and coworker instrumental support, such that when self-esteem was low, the relationship between authenticity and coworker support was significantly weaker. Additionally, narcissism moderated the relationship between authenticity and employee effectiveness such that when narcissism was low, the relationship between authenticity and effectiveness was positive, but when narcissism was high, the relationship became negative.