**1. Introduction**

The Internet of Things (IoT) refers to the process of connecting everyday physical objects to the internet, from common household objects (lighting, appliances, etc.) to healthcare assets (such as medical devices), as well as wearables, smart devices and even smart cities.

These IoT-connected physical objects are visible within the created network itself, allowing them to be consulted and/or acted upon.

The great advantage of the IoT, which leads to its enormous importance today, centres on the ease of connecting new objects to this network. The interest in this technology is being increased year by year, as shown in **Figure 1**.

A few years ago, in order to connect a device to the network, it was necessary to deploy a multi-layered infrastructure to access its information. Nowadays, however, there are open-access projects that present a free and extensive network where end users can directly connect their objects (only needing a connection modem in the object itself).

With multiple connected objects over a large area, there is great potential for projects that focus on the population's well-being when applied to smart cities. This opens up endless possibilities, but not without challenges and concerns. Many of the latter focus on the devices and network's security and devices and how a malicious user can alter the information or undermine privacy.

All these issues and possibilities are addressed in the various chapters of this book, which attempt to cover all areas of the IoT.

So, the main aim of this introductory chapter is to serve as a justification for the book itself, presenting hard facts and data that prove the evolution of the use and deployment of IoT systems in society. To this end, a literature review will be carried out to show the increase in publications related to the subject in recent years.
