When you are in charge of building software from the ground up, software security can be encouraged through the use of secure software development methodologies. However, how can you measure the security of a given piece of software that you didn’t write yourself? In other words, when looking at two executables, what does “a is more secure than b” mean? This paper examines some approaches to measuring software security, and reccommends that more organisations should employ the Building Security In Maturity Model (BSIMM).