Symbiosis, where organisms of different species live closely together, is ubiquitous in our world. It is thought that all multicellular organisms engage in symbiosis, in relationships where the symbiont's effect on the host ranges from beneficial (mutualism) to neutral (commensalism) to harmful (parasitism), and can even switch between mutualism and parasitism, depending on environmental conditions (conditional mutualism). Symbiosis can also have a large, though generally beneficial, impact on the symbiont's fitness, and can further affect third parties who interact with the host or symbiont. The manner in which symbionts are transmitted between hosts can affect not only the distribution of symbionts (and hosts) but also selection on the costs and benefits of the interaction. For example, symbionts are that are transmitted from parent to offspring (vertical transmission) are predicted to evolve to benefit their hosts, while symbionts that are transmittted between unrelated individuals (horizontal transmission) are predicted to evolve intermediate virulence that maximizes their ability to infect new hosts. Understanding transmission evolution is thus important to predicting the ultimate fate of symbioses. I investigate three aspects of transmission evolution. First, since transmission evolution is known to depend on whether it is under host or symbiont control, I used phylogenetic methods to estimate control of transmission in the symbiosis between cool-season grasses (subfamily Pooideae) and their fungal endophytes (genus Epichlo e). I found a signal of joint control of transmission, suggesting that the interaction of host and symbiont traits may determine the transmission mode. Second, while a great deal of theory exists about transmission evolution under different types of control in parasitic and mutualistic symbioses, less is known about transmission evolution in conditional mutualisms. These symbioses pose a problem for hosts, who benefit from acquiring the symbiont only in environments where it is beneficial. I modeled transmission evolution in a context-dependent interaction where symbiont quality varied in space and found that the aspect of host fitness the symbiont affects determines host transmission evolution. When the symbiont affects lifespan, but not fecundity, hosts are able to use horizontal transmission to contain the symbiont to the location where it is beneficial. Because environments can vary in time as well as space, I lastly modeled transmission evolution in a conditional mutualism in a spatially and temporally variable environment. In this case, I found that environmental synchronicity could allow hosts to evolve vertical transmission at high newborn host dispersal rates, where ordinarily parent and offspring environments would be too uncorrelated to allow for vertical transmission as a method of symbiont containment. I also found an emergent trade-off in hosts between horizontal and vertical transmission, suggesting that physiological constraints are not required to produce apparent constrains on the total amount of transmission.