  We prove three theorems in general relativity which rule out classical scalar
hair of static, spherically symmetric, possibly electrically charged black
holes. We first generalize Bekenstein's no--hair theorem for a multiplet of
minimally coupled real scalar fields with not necessarily quadratic action to
the case of a charged black hole. We then use a conformal map of the geometry
to convert the problem of a charged (or neutral) black hole with hair in the
form of a neutral self--interacting scalar field nonminimally coupled to
gravity to the preceding problem, thus establishing a no--hair theorem for the
cases with nonminimal coupling parameter $\xi<0$ or $\xi\geq {1\over 2}$. The
proof also makes use of a causality requirement on the field configuration.
Finally, from the required behavior of the fields at the horizon and infinity
we exclude hair of a charged black hole in the form of a charged
self--interacting scalar field nonminimally coupled to gravity for any $\xi$.
