The understanding on the principles and applications of analogue electronic devices and circuits are very crucial to the beginners in electronics or electrical engineering study. The prime objective of this book is to provide entry level knowledge of electronics. The prerequisite for this book is basic knowledge of electrical circuit principles. This book is about the study of electronic devices and circuits. The chapters in this book progress from an introduction to the electronic devices and followed by theory on semi–conductors, diodes and diodes application, BJT and FET transistors, the concept of gain and amplifiers, transistor as a switch and thyristor. The information, theory, concepts and calculations are presented in the book in simple manner to assist the student to grasp and understand on how electronic devices function and being used in circuits. It should be noted that modern electronics is a vital and exciting field, and studying it should be the same.