In this chapter we introduce the basic building blocks of the digital logic systems found in modern aircraft. We begin with a review of the different types of logic gates before moving on to explore their use in several typical aircraft applications. The chapter also provides a brief overview of coding systems and the logic arrangements that are used to generate and convert the codes that are used to represent numerical data. The chapter provides an introduction to aircraft bus systems and concludes with a brief overview of the architecture and principal constituents of simple computer systems.