This chapter has discussed one of the most complex phases of an attack. Not all of the techniques used here are complex though. As has been said, there are two techniques; horizontal and vertical privilege escalation. Some attackers will use the horizontal privilege escalation methods because they are less tasking and easier to perform. However, veteran hackers who have a good understanding of the systems that they target use vertical privilege escalation methods. This paper has gone through some of these privilege escalation methods. It was clear from most methods that hackers had to target legitimate processes and services in order to escalate privileges. This is because most systems are built using the least privilege concept. Users are purposefully given the least privileges that they require to accomplish their roles. Only the legitimate services and processes are given high-level privileges and, therefore, attackers have to compromise them in most cases.