Wavelength-selective channel switches (WSS) have been instrumental in realizing dynamic transparent optical networks, where WDM channels are switched, dropped and added at network nodes. While the technology for realizing WSS has been around for a decade, developments in the field have never ceased and have followed the advancements in modern optical communications. Some of these developments are evolutionary, such as support for higher port counts and twin-packing of WSS in an effort to cost-reduce its deployment. Others are more revolutionary, enabling new functionality at the optical transport layer, such as flexible bandwidth switching in support of super-channel transmission, and high-resolution optical multiplexing of new modulation formats such as orthogonal frequency division multiplexing (OFDM) and Nyquist-WDM. We review the future trends in WSS technology, demonstrating the advanced functionality required in support of flexible/elastic optical networks, and predicting how far forward the technology might scale.