  In this chapter we consider two formation channels for blue straggler stars:
1) the merger of two single stars via a collision, and 2) those produced via
mass transfer within a binary. We review how computer simulations show that
stellar collisions are likely to lead to relatively little mass loss and are
thus effective in producing a young population of more-massive stars. The
number of blue straggler stars produced by collisions will tend to increase
with cluster mass. We review how the current population of blue straggler stars
produced from primordial binaries decreases with increasing cluster mass. This
is because exchange encounters with third, single stars in the most massive
clusters tend to reduce the fraction of binaries containing a primary close to
the current turn-off mass. Rather, their primaries tend to be somewhat more
massive and have evolved off the main sequence, filling their Roche lobes in
the past, often converting their secondaries into blue straggler stars (but
more than 1 Gyr or so ago and thus they are no longer visible today as blue
straggler stars).
