Sperm donation is a widely accepted and increasingly common practice. In the standard case, a sperm donor sells sperm to an agency, waives his parental rights, and is absolved of parental responsibility. We tend to assume that this involves no problematic abandonment of parental responsibility. If we regard the donor as having parental responsibilities at all, we may think that his parental responsibilities are transferred to the sperm recipients. But, if a man creates a child accidentally, via contraception failure, we (...) tend to assume that the man does indeed have parental responsibilities. Assessing these contrasting conclusions requires a theory of parental responsibility. I analyze prevalent theories of what makes someone parentally responsible and show that none of these theories can withstand scrutiny. I propose a new theory of parental responsibility, which, I argue, is more plausible than the alternatives. My theory of parental responsibility is based on our ownership and control over hazardous materials, namely, our gametes. I show that neither my theory, nor the theories I reject, can support our contrasting intuitions. I conclude that sperm donors are fathers, with parental responsibility. I argue that the alternative conclusion, that neither sperm donors nor accidental fathers are parentally responsible for their resulting offspring, is less plausible. I then consider whether parental responsibility can be transferred and argue that it is far from clear that it can. Finally, I address objections and consider some practical implications of these views. (shrink)