Swarm robotics is a relatively new technology that is being explored for its potential use in a variety of different applications and environments. Previous emerging technologies have often overlooked security until later developmental stages, when security has had to be undesirably (and sometimes expensively) retrofitted. This paper compares swarm robotics with related technologies to identify their unique features where existing security mechanisms can not be applied. We then review some of the emerging applications where ineffective security could have significant impact. We conclude by discussing a number of security challenges for swarm robotics and argue that now is the right time to address these issues and seek solutions. We also identify several idiosyncrasies of swarm robotics that present some unique security challenges. In particular, swarms of robots potentially (i) employ different types of communication channels (ii) have special concepts of identity, and (iii) exhibit adaptive emergent behaviour which could be modified by an intruder. Addressing these issues now will prevent undesirable consequences for many applications of this type of technology.