Most studies on casual sexual relationships and experiences (CSREs) are quantitative and focus on characteristics of individuals who experience them. There has been an increase in qualitative research on CSREs that have revealed new insights into how they are experienced among young adults. To synthesize qualitative knowledge on how CSREs are experienced, a qualitative metasynthesis was carried out using 13 studies published up to June, 2015, yielding seven conceptual categories. Opportunities and choices regarding CSREs were organized according to biographical context, socioenvironmental context, and peer norms. Expectations of CSREs related to lack of communication, emotional, and sexual commitment or accountability. Implicit rules allowed maintaining boundaries between sex and emotion. Thus, CSREs can satisfy various needs related to sex and intimacy. Sex in CSREs was reported as pleasure-centred, accessible, and improved when experienced in association with intimacy. However, intimacy and gender roles interfered with condom use. The sexual double standard and gender roles limited women's agency and enjoyment of sex. Expectations for the lack of intimacy and communication conflicted with developing unrequited feelings, resulting in self-blame and attempts to suppress emotions. While partners may have a hard time defining their relationship over time, some CSREs involved developing friendships or romance, and ruining friendships. The results indicate a clash between expectations, rules, and experiences of CSREs. The lack of symbolic markers for the meaning or status of CSREs as they develop can be seen as a consequence of the complexity of these relationships.