In this chapter the goal is to give a brief overview of the different instruments, systems, and subsystems that one can find in a typical aircraft. First, in Section 5.1, the focus will be on the instruments. Notice that modern aircraft are becoming more and more sophisticated and classical instruments are being substituted by electronic displays. Aircraft systems are briefly analyzed in Section 5.2. Again, classical mechanical (pneumatic, hydraulic) systems are being substituted by electronics. Therefore, in modern terminology, the discipline that encompasses instruments (as electronic displays) and electronic systems About the CourseComedy is the funniest traffic school in California! Take the online traffic school course from the comfort of your own home or office, not in a distant classroom. is referred to as avionics. Nonetheless, still many small aircraft use instruments and also some important aircraft systems are not based on electronics, e.g., fuel system. A introductory reference is Franchini et al. [1], in which this chapter is inspired. Thorough references on aircraft systems are Moir and Seabridge [4], Kossiakoff et al. [2], Tooley and Wyatt [5], and Langton et al. [3].