Internet of Things (IoT) is a transformative approach of interconnecting many heterogeneous entities that are embedded with the capabilities of sensing, computing and communication. These entities are characterized by small size, less computation power, little memory and limited battery. The presence of large heterogeneity and openness introduces many security breaches. To provide security for these constrained devices, traditional cryptographic primitives and strategies are not suitable. This paper presents a review on research advances related to the security aspects of the Resource constrained IoT environments. In this paper, Architecture of the IoT and its Applications are first discussed. Secondly, classification of the constrained devices is defined. Thirdly, Security objectives in terms of confidentiality, Authentication and integrity are discussed. Later, recently proposed security approaches suited for the constrained environments are discussed. Finally, the hardware and software platforms needed for IoT realization are discussed. © 2018, Institute of Advanced Scientific Research, Inc.. All rights reserved.