Everybody know these words. You say it when you have to check a microphone which were plugged into some sound system. It’s just test if the microphone is working well and the spoken words are reflected by the amplifier. It’s a simple test to check the functionality of the microphone or the whole sound system. Why should I write about sound systems? No, I don’t try to learn playing a guitar or any other instrument, I like to share some thoughts about software testing. After writing a post about Valgrind, I had the feeling that I should try to show some ways of testing software. In the following post I will analyze what can be tested, what is useful and which tools are available. I don’t aim to make a full overview of software testing itself, I just like to highlight some tools (or thoughts) which are useful and every developer should know of.