Guide for developing STEM applications and educational activities with Arduino. In today's era, wherever we look, we will find hidden mechanisms that control certain conditions and act accordingly. These mechanisms are usually based on microcomputers or microcontrollers and are found inside household appliances, personal electronic devices we use daily (digital wristwatches, mobile phones, tablets, etc.), even inside our cars, which in some cases can park or even drive "on their own." All these seem marvelous because they appear to happen by themselves. However, if we ponder a bit, we will realize that behind them lies a human mind, which has given the appropriate commands to these mechanisms to act in this way. The constructions we can create with Arduino are numerous and for various purposes. Hobbyists can build remote-controlled cars, students and pupils can create various setups that can take measurements, light up LEDs when the brightness in the area is low, activate mechanisms, and much more. The book you hold in your hands follows the spiral curriculum method of teaching, as formulated in 1960 by the American psychologist Jerome Bruner. It contains a plethora of examples, most of which can be practically implemented, thus giving the reader the opportunity to become familiar with the Arduino C++ programming language until they are finally able to create their own applications. It is suitable for beginners as well as advanced users, for middle-high school students as well as university students of higher and tertiary educational institutions, for amateurs in the field as well as professionals. Accompanying material The source code of the exercises, the answers to all the questions, the solutions to all the exercises, as well as videos of the applications are provided for free via the Internet. Book contents Introductory concepts, algorithmic and programming, Electrical and electronic circuits, applications for beginners, Applications for advanced users, useful elements.