This is a very old and famous quote by Abraham Lincoln. The same applies to the amount of time spent in gathering as much information as possible prior to performing any security assessment. Unless, and until, you know your target inside and out, you will never succeed in performing its security assessment. It's crucial to have a 360-degree view of the target and gather all possible information about it through all available sources.

Once you are confident that you have gathered enough information, then you can very effectively plan the actual assessment. Information gathering can be of two types, as discussed in the following sections: passive information gathering and active information gathering.