In this chapter, we will have an overview of a quantum computer from an engineer’s perspective. We will first look at the basic operations of a quantum computer, namely, qubit initialization, qubit manipulation, and qubit readout. We then introduce and discuss DiVincenzo’s criteria which set the minimal requirement for a technology to build a quantum computer. Then we will use a superconducting qubit quantum computer as an example to trace the propagation, attenuation, modification, and amplification of the microwave signals to gain a deeper understanding of the operating principles of a quantum computer and the critical role of classical and microwave electronics in a quantum computer. We will also discuss the scaling of a quantum computer.