We present a precise description IoT-LAB. IoT-LAB provides a very large scale infrastructure facility suitable for testing small wireless sensor devices and heterogeneous communicating objects. IoT-LAB features over 2700 wireless sensor nodes spread across six different sites in France. Nodes are either fixed or mobile and can be allocated in various topologies throughout all sites. A variety of wireless sensors are available, with different processor architectures (MSP430, STM32 and Cortex-A8) and different wireless chips (802.15.4 PHY @ 800 MHz or 2.4 GHz). In addition, “open nodes” can receive custom wireless sensors for inclusion in IoT-LAB testbed. IoT-LAB's main and most important goal is to offer an accurate open access multi-users scientific tool to support the design, development tuning, and experimentation of real large-scale sensor network/IoT applications. The hardware and software architectures that allow to reserve, configure, deploy embedded software, boot wireless sensor nodes and gather experimental data and monitoring information are described in detail. We also present demonstration examples to illustrate the use of the IoT-LAB testbed. IoT-LAB is part of the FIT experimental platform, a set of complementary components that enable experimentation on innovative services for academic and industrial users. The project -Future Internet of Things (FIT) – is one of 52 winning projects from the first wave of the French Ministry of Higher Education and Research’s “Équipementsd’Excellence” (Equipex) research grant programme. The FIT project gives French Internet stakeholders a way to experiment with mobile wireless communications, both on network and application layers, thereby accelerating the design of advanced networking technologies for the Future Internet.