  The physical processes that determine the properties of our everyday world,
and of the wider cosmos, are determined by some key numbers: the 'constants' of
micro-physics and the parameters that describe the expanding universe in which
we have emerged. We identify various steps in the emergence of stars, planets
and life that are dependent on these fundamental numbers, and explore how these
steps might have been changed, or completely prevented, if the numbers were
different. We then outline some cosmological models where physical reality is
vastly more extensive than the 'universe' that astronomers observe (perhaps
even involving many 'big bangs'), which could perhaps encompass domains
governed by different physics. Although the concept of a multiverse is still
speculative, we argue that attempts to determine whether it exists constitute a
genuinely scientific endeavor. If we indeed inhabit a multiverse, then we may
have to accept that there can be no explanation other than anthropic reasoning
for some features our world.
