IoT applications provide advantages in various domains from smart cities, factories of the future, connected cars, home automation, e-health to precision agriculture. This fast-growing ecosystem is leading IoT towards a promising future. However, IoT systems expansion opportunities are not straightforward. A set of challenges should be overcome to enable IoT mass-scale deployment across various domains including interoperability, complexity, and scalability issues. This talk aims at giving an overview of these challenges. Recent international standardization and R&D initiatives will be investigated. Future directions will be highlighted.