Programmable Logic Controller (PLC) was originally designed and developed in 1968 by General Motors for replacing hardwired relay logic circuit. PLCs are available with a wide range of specifications and performance. Hence, it is difficult to classify them in a straightforward way. However, classification of PLCs can be made based on their various parameters like structure, functionality, I/O handling capacity, etc. In process automation, from robotic arm movements in the automotive industry to safety aspects of chemical plants, PLCs play a major role. This chapter describes the features of the PLC that would satisfy various requirements in process automation. Ladder diagram (LD) is the most popular form of PLC programming languages. Simplicity and easy understanding are the key reasons for the wide acceptance of LD. PLC communication lends an effective and efficient way for industries with large number of I/O devices distributed over a large area to communicate with each other.