The outsourcing of the design and manufacturing of Integrated Circuits~(ICs) poses a severe threat to our critical infrastructures as an adversary can exploit them by bypassing the security features by activating a hardware Trojan. These malicious modifications in the design introduced at an untrusted fabrication site can virtually leak any secret information from a secure system to an adversary. This paper discusses all three different hardware Trojan models, such as combinational, sequential, and analog Trojans. We provide a survey of the recent advancements in Trojan detection techniques classified based on their applicability to different Trojans types. We describe a practical approach recently developed using the characterization of Electro-Optical Frequency Mapping~(EOFM) images of the chip to detect a hardware Trojan by identifying malicious state elements. This survey also presents open problems with Trojan detection and suggests future research directions in hardware Trojan detection.