High spectral efficiency modulation is key to addressing the optical network capacity challenge. Typical implementations of transmitters for advanced modulation formats use high-resolution silicon digital-to-analog converters, complemented by high bandwidth linear amplifiers to drive the electro-optic modulator. To overcome the limitations of this setup, both in speed and complexity, Bell Labs has proposed and realized an alternative approach based on a multi-level driver which operates at symbol rates up to 50 GBaud with swings up to 2×2 volts, sufficient to drive various common electro-optic modulators (Vπ ∼3-4 V). Over the past two years, multi-level drivers have enabled various Bell Labs optical system experiments relying on the high signal quality achieved and the rate flexibility offered. The circuit architecture, its main features, and the various system experiments are summarized. © 2013 Alcatel-Lucent.