ESP8266 can be used with the official EspressIf's SDKs that contain the NonOS and FreeRTOS SDKs writing code in C/C++, but other companies and people add other programming languages to it, such as Lua, Javascript, or MicroPhyton.

This book will use the NonOS SDK and as a development IDE, the Arduino IDE. In this part you will download the Arduino Software (IDE), configure it and install the ESP8266 SDK.