Handover is one of the enhancement mechanisms that plays a central role in heterogeneous wireless networks. Designing a handover algorithm for heterogeneous networks becomes an essential issue since it is responsible for providing better Quality of Service (QoS) and higher level of Quality of Experience (QoE) to users. Similarly, the proposed algorithms must fulfill the user's QoS requirements and must also provide seamless roaming across different networks. This paper presents an overview of some key issues pertaining to the design of Vertical Handover (VH) in 4G and 5G wireless networks. The VH measurements and decision mechanisms are studied in relation to network types and frequency used mechanisms. Moreover, the advantages and drawbacks of the existing works are investigated in detail and the challenging issues of the next generation of wireless networks are presented as well.