One of the first life history characters that students of bryology learn is the bryophyte life cycle. Roughly half of the bryophytes have separate sexes (dioicous) and half are hermaphroditic (monoicous). This variation is important for taxonomic purposes but also has major consequences for nearly every aspect of bryophyte biology. For example, dioicous species generally reproduce less frequently than monoicous species (Longton & Schuster 1983) but harbor more genetic variation than monoicous species (Eppley et al. 2006). More broadly, however, the maintenance of both sexual systems at nearly equal frequencies in the bryophytes is quite unusual among the eukaryotes. This situation makes bryophytes an important model system for studying the evolutionary consequences of changes in sexual system. Bryophytes have a long history as model systems in cytology and genetics. Sex chromosomes in plants were first discovered in the liverwort Sphaerocarpus donnellii (Allen 1917), and the term ‘‘heterochromatin’’ was coined for the dark staining heteromorphic pair of chromosomes in many bryophytes (Heitz 1932). Bryophytes have long served as model genetic systems because the dominant part of the life cycle (i.e., the gametophyte) is haploid, and the phenotype of the plant directly reflects its genotype without the masking of dominance (Allen 1919; Ashton & Cove 1977; Engel 1968; von Wettstein 1924). More recently, efficient gene targeting via homologous recombination was discovered in Physcomitrella patens (Kammerer & Cove 1996; Schaefer & Zryd 1997; Strepp et al. 1998) and Ceratodon purpureus (Brucker et al. 2005; Mittmann et al. 2009; Trouiller et al. 2007). With this tool researchers can construct powerful tests for associations between genes and traits. In addition, the genome of P. patens has now been completely sequenced (Rensing et al. 2008), and sequencing projects for C. purpureus (http://www.jgi.doe.gov/ sequencing/why/ceratodon.html), Phaeoceros laevis (http://ldl.genomics.org.cn/page/searchplant. jsp?speciesname5phaeoceros), and Marchantia polymorpha (http://www.jgi.doe.gov/sequencing/ why/99191.html) are now underway. Together the natural variation in sexual system and the potential for laboratory study make bryophytes ideal for studying the causes and consequences of changes in sexual system. The purpose of this perspective is to highlight areas where interdisciplinary research can provide new insights into the evolution of bryophyte sexual systems. We have structured the perspective around three major evolutionary transitions: 1) the evolution of dioicy; 2) the evolution of sex chromosomes; and 3) the evolution of monoicy. We first provide a brief outline of the evolutionary theory underlying each of these phenomena. Since much of this theory was developed for angiosperms, we suggest how it can be applied to bryophytes. Second, we indicate how 3 Corresponding author’s e-mail: stuartmcdaniel@ufl.edu DOI: 10.1639/0007-2745-115.1.1