Algorithms are well-defined sequences of operations that are executed to solve a problem and/or to compute a result. Today algorithms are designed to be executed by automatic machines. They are the main abstract engines in the automating processes we daily use. In fact, algorithms automate many decisions that have an impact on people life. This chapter introduces the concept of algorithm, its principles and features. Then it discusses examples of algorithms and how they are designed to obtain software programs that are run on computers.