The order Alismatales is a mostly aquatic group of monocots that displays substantial morphological and life history diversity, including the seagrasses, the only land plants that have re-colonized marine environments. Past phylogenetic studies of the order have either considered a single gene with dense taxonomic sampling, or several genes with thinner sampling. Despite substantial progress based on these studies, multiple phylogenetic uncertainties still remain concerning higher-order phylogenetic relationships. To address these issues, I completed a near-genus level sampling of the core alismatid families and the phylogenetically isolated family Tofieldiaceae, adding these new data to published sequences of Araceae and other monocots, eudicots and ANITA-grade angiosperms. I recovered whole plastid genomes (plastid gene sets representing up to 83 genes per taxa) and analyzed them using maximum likelihood and parsimony approaches. I recovered a well supported phylogenetic backbone for most of the order, with all families supported as monophyletic, and with strong support for most inter- and intrafamilial relationships. A major exception is the relative arrangement of Araceae, core alismatids and Tofieldiaceae; although most analyses recovered Tofieldiaceae as the sister-group of the rest of the order, this result was not well supported. Different partitioning schemes used in the likelihood analyses had little effect on patterns of clade support across the order, and the parsimony and likelihood results were generally highly congruent. I also used the inferred phylogeny of Alismatales to study the loss of the mostly plastid-encoded NADH dehydrogenase enzyme complex in the order. This enzyme is hypothesized to be involved in mitigating photooxidative stress by inducing chlororespiration. The inclusion or exclusion of ndh pseudogenes had little impact on the main phylogenetic results. Previous work hypothesized three independent losses/pseudogenization events within the core alismatids, which I confirmed here. I also inferred an additional loss in Tofieldiaceae, the first example in unsubmerged species of Alismatales. The repeated loss of plastid NADH dehydrogenase may spur future research into the physiological bases of the loss.