Solitude is a normative human experience throughout the lifespan. Notwithstanding, there continues to be considerable debate regarding the implications of spending time alone. In this chapter, we review theory and research pertaining to the links between solitude and well-being. We begin with an overview of the conceptual and methodological approaches to the psychological study of solitude. This is followed by a brief summary of the evidence demonstrating the various potential costs of spending excessive time alone. We then closely examine the supposition that solitude can also serve beneficial functions, including some promising areas worthy of future investigation. Our goal is to begin to untangle the so-called paradox of solitude: Although spending time alone is believed to serve self-enhancing functions, solitude is often experienced as undesirable and painful.