Thinking is a behavior, which often implicit and hidden in which symbols (images, ideas and concepts) are ordinarily employed. Thinking is a problem-solving process in which we use ideas or symbols in place of overt activity. Thinking is increasingly complex and difficult concept to discuss and explain in psychology. Thinking can be grounded as intermediate stage between a stimulus and a response. Developmentally, a child thinking is much simpler and progress to complex thinking as child grows up. Types of thinking, level of thinking, building blocks of thought/thinking, factors influencing thinking process, relationship with language and communication, and thinking and sickness are also discussed in this chapter. In this chapter, short essay type questions and multiple choice questions along with their answer key are also provided.