For high speed and low power ASIC designs, focus is on minimizing the delays, signal integrity, power integrity, through techniques like careful routing, impedance matching, and power delivery network optimization. By considering this we always plan the strategy to have the low power, high speed, and the DFT friendly designs. To design the efficient digital circuits, we need to understand the constraints and basic digital elements. This chapter is focused on the basic fundamentals of the digital circuit and the basic logic gates and use of them in the design.