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Ant-associated behavior has evolved rampantly among various groups of Arthropoda, and has arisen in at least 34 families of beetles. Due to the amazing morphological modifications and different kinds of interactions that occur within myrmecophilous (ant-associated) beetles, authors have predicted that myrmecophily has evolved in a step-wise fashion from casual, facultative associations to closely integrated, obligate interactions. In this dissertation, myrmecophily within the Coleoptera is reviewed, and known behaviors, ant-beetle interactions, and associated morphological adaptations are discussed. In order to better understand how myrmecophily has evolved, two groups of beetles are studied in a phylogenetic context. A cladistic analysis of 40 species of the myrmecophilous scarab genus, Cremastocheilus Knoch is presented. Characters related to a myrmecophilous habit are largely informative, especially those characters related to the glandular trichomes (clusters of setae typically associated with exocrine glands). Two of the five previously recognized subgenera, C. (Myrmecotonus) and C. (Anatrinodia) are synonymized with the subgenus C. (Cremastocheilus). Even though behavioral information is only known for a few species, the resulting phylogeny indicates that monophyletic subgenera are largely associated with the same ant hosts, although specific interactions with ant hosts can vary even in closely-related taxa. In addition, a separate cladistics analysis of the spider beetles based on morphology is presented. The monophyly of previously proposed suprageneric groups are investigated, and eight tribes of spider beetles (four unnamed basal tribes, plus more derived Gibbiini, Ptinini, Sphaericini) are recognized to capture three unique monophyletic groups of spider beetles. Of eight myrmecophilous spider beetle genera, only one genus (Gnostus) can be placed, all other myrmecophilous representatives remain unplaced in phylogeny. Based on this analysis, myrmecophily has evolved independently in four lineages, with nearly all genera appearing basal in spider beetle phylogeny. Based on these findings, obligate myrmecophily has evolved in four different groups. Among the Old World taxa (South African, Australian), and based on morphology, antennae may have evolved in a step-wise fashion, from less specialized (normal, 11-segmented antennae) to more highly specialized (reduced, with fusion of segments); however, pronotal trichomes are present in all related taxa. Among New World taxa, the presence of unique myrmecophilous adaptations indicates rapid evolution of obligate ant-associated behavior and morphology, rather than step-wise evolution from casual or facultative to obligate associations. Finally, two new genera of Dominican amber spider beetles are described. The new genus Electrognostus may indicate a transition from a normal Ptinus-like spider beetle to a myrmecophilous type like Gnostus, but based on the phylogenetic analysis it can currently be placed within the Ptinini tribe.