Parallel Wireless Communications is an emerging area of research that enables mobile clients to access and utilize multiple wireless networks concurrently. This paper explores concurrent bandwidth aggregation techniques to enable parallel wireless communications on mobile devices. Concurrent aggregation can be performed using wireless modems on one device or cooperatively across devices within wireless proximity of each other.. The aggregation can be achieved on the application-layer, transport layer like MultipathTCP, or on layers below IP such as the RLC layer. These schemes are compared. In general, aggregation on the lower layer is closer to the client device and easier to adapt to the varying channel and loading conditions. However, lower layer aggregation needs more changes to wireless access stratum infrastructure. Aggregation above the IP layer is simpler to implement. A feasibility study of MultipathTCP aggregation over multiple WWANs is presented. The aggregation scheme on the RLC layer in evolved HSDPA is also presented, including protocol design and performance evaluation.