Software defined networks are based on the principle of a centralized control plane separated from the network forwarding or switching plane that it controls. The switching plane can be heterogeneous, composed of network elements from multiple vendors, and it can provide distinct services with different characteristics, configurations, and control at the packet and/or optical layers. Abstracting the control plane from the network elements allows network-platform- specific characteristics and differences that do not affect services to be hidden. In addition, software defined networking (SDN) is based on the principle that applications can request needed resources from the network via interfaces to the control plane. Through these interfaces, applications can dynamically request network resources or network information that may span disparate technologies. For instance, the application layer can dynamically request and obtain network resources at the packet flow, circuit, or even optical level based on application layer requirements. Current SDN implementations focus on Ethernet switching primarily for data center resource optimization. This article reviews the benefits and challenges of extending SDN concepts to various transport network architectures that include optical wavelength and fiber switches, circuit switches, and sub-wavelength optical burst switches. Control plane implementations for optical networks are more complex since they must account for physical constraints including optical signal reachability, bandwidth availability and granularity, light path routing, and light path reconfiguration speed. The longterm goal is to apply SDN concepts across multi-layer multivendor networks in order to support a unified control structure.