This talk will give an introductory lecture on the basic principles of quantum computers. After a brief introduction to quantum computing, it will focus on two physical platforms for quantum computers: semiconductor spin qubit and superconducting qubit systems. Then, it will explain how to implement the essential elements of quantum computing - e.g., qubit, gate operations, and measurement - in the physical systems. Applications and limitations of current noisy-intermediate-scale-quantum (NISQ) devices and future outlook for scalable, practical quantum computers will also be discussed.