An ad hoc network is a collection of wireless mobile nodes dynamically forming a network without the use of any existing stationary network infrastructure. The network can be multi-hop and mobile; there is no central controller and packet transmissions are typically unsynchronized. The efficiency of the medium access control (MAC) protocol to coordinate the access to the shared radio medium is critical. Carrier sense multiple access (CSMA) protocols are typically used. However, their efficiency is limited when the load and the level of contention is high. This thesis proposes use of multichannel CSMA protocols to reduce contention on the wireless medium. Though the aggregate capacity with a multichannel scheme is the same as a single channel, contention per channel is now lower and thus channel access is more efficient. We show that only a handful of channels provide optimum performance as with too many channels per-channel bandwidth is too low that affects performance adversely. Since the number of channels are much lower than the number of nodes, effective channel selection schemes are needed. We propose a receiver-based channel selection (RBCS) scheme that selects channel based on interference levels on different channels at the receiver. We implement this technique as an extension of IEEE standard 802.11 MAC protocol (which is a single channel protocol) on a network simulator. We show that it provides superior delay performance at high loads compared to single channel, as well as other, previously studied, channel selection schemes, such as selcting a random free channel or selecting channel based on sender-side signal power. As a final contribution, we study the effect of multichannel CSMA protocols for multipath routing on ad hoc networks. With use of single channel CSMA, ''route coupling'' can exist for multipath routes. This means that routes can form in radio neighborhood, and their transmissions can interfere with each other, preventing multiple routes to be used concurrently. Thus, the load balancing advantages of multiple paths are lost. We show that the use of multichannel CSMA protocols as above can remarkably improve the effectiveness of the multipath routing by providing more diversity.