Penetration testing is an effort to attack a system using similar techniques and tools adopted by real hackers. The ultimate goal of penetration testing is to call to light as many existing vulnerabilities as possible, then come up with practical solutions to remediate the problems; thus, enhance the system security as a whole. The paper introduces concepts and definitions related to penetration testing, together with different models and methodologies to conduct a penetration test. A wide range of penetration testing state-of-the-art, as well as related tools (both commercial and free open source available on the market) are also presented in relatively rich details.