The Chirped Return to Zero modulation format is now widely deployed in Tyco Telecommunication's undersea terminal equipment. This modulation format has allowed for an increase in capacity, system reach and performance of undersea telecommunications systems. Introduction - The Chirped Return to Zero modulation format (or CRZ) can provide a combination of increased capacity, system reach, and performance for long-haul undersea cable systems. The performance of a long-haul undersea cable system can be degraded by a combination of the transmission fiber's nonlinear index and chromatic dispersion. R&D efforts to understand these impairments have lead to many important transmission improvement techniques such as dispersion management, new transmission fibers, FEC, and modulation formats. In this paper we review the basics of the CRZ modulation format, add some historical perspective on this (and other) modulation formats, and then give simple time/frequency domain arguments for why the CRZ modulation format is useful. Historical Perspective - Today the subject of modulation formats is hot, but it is not new. Modulation formats have been debated over the entire history of optical fiber communications. Early optical fiber transmission demonstrations used the Non Return to Zero (NRZ) modulation format because it was easy to generate and detect (1). In the 1980's many fiber optic research organizations studied coherent optical communication techniques (including DPSK) to increase receiver sensitivity and increase receiver selectivity. (2) Work on coherent optical communications slowed down dramatically in the early 1990's with the demonstration of the Erbium-doped Fiber amplifier. Long EDFA-based systems started with the NRZ modulation format. Later, formats such as RZ and CRZ took hold for today's 10Gb/s systems. Recently, there has been a strong interest in the DPSK format because of the 3 dB receiver advantage (3), and in bandwidth reduced formats such as duobinary.