The major focus of this dissertation is the ancestor worship that is inferred to have been practiced in the multiethnic Middle Sicán theocratic state (AD 950-1100) that prospered on the northern North Coast of Peru. The major objective is twofold: (1) demonstrating by archaeological means that ancestors were indeed worshipped in the Middle Sicán society and (2) elucidating the nature and role of the inferred ancestor cult and associated rituals and ceremonies. Ancestor (and the veneration of it) is one of the themes that have the deepest roots in the anthropological thoughts; nevertheless, many archaeologists have uncritically invoked ancestor veneration without sufficient theoretical underpinning and empirical support, to the point that James Whitley (2002) decried "too many ancestors." This dissertation thus begins with a review of the earlier anthropological discoveries and theoretical debates on what ancestor is and who becomes an ancestor, including the cases in the Andes. Based on this review of previous studies, it is hypothesized that the select members of deceased Middle Sicán elites were transformed into an ancestor through a series of prescribed processes. This hypothesis is examined in terms of the five possible material correlates of the inferred Sicán ancestors extracted from the regional archaeological database of the study area accumulated by the Sicán Archaeological Project (SAP) for the last three decades. The role of the inferred Middle Sicán ancestor cult is approached from the ideological perspective. It is inferred that the ancestor cult was employed by the ruling group as an ideological and political means to justify the existence and extension of social hierarchies and inequalities and thus targeted at wider populations different in genealogical origins as opposed to family or lineage members. This study focuses attention on the food preparations and consumptions documented by a test excavation at the principle plaza of the Sicán capital, "Great Plaza," adjacent to the inferred ancestral tombs and hypothesizes that the commensality among the living and the dead during feasts there served not only to commemorate the inferred ancestors, but also to bring together people in different social tiers and to consolidate the highly stratified, multiethnic Middle Sicán society. Two excavations at the ceremonial core of the Middle Sicán state capital, one at the Huaca Loro West Cemetery in 2006 and the other at the Great Plaza in 2008, provide varied lines of evidence that support the above two hypotheses. The results suggest that ancestor worship was indeed practiced during the Middle Sicán Period. By maintaining and monopolizing the ritual access to the Sicán Deity through their ancestors, the Sicán elites reproduced their religious and political power and retained the legitimacy of their social status. Concurrently, the Sicán elites consciously employed their ancestor cult for social integration. After the Middle Sicán Period, these ancestors seem to have retained their spiritual viability even after the later Chimú Empire took the control of this region. If not recognized as the Sicán anymore, they were remembered and honored by the living for over four centuries. On the basis of the merits of traditional approach (e.g., the study of architecture, iconography, bioarchaeology, and ethnohistory and ethnography in the Andes), this study gives primacy to the direct focus on the material residues and relational contexts and patterns of ritual activities and studies their change and stability through time in relation to other historical contingencies. The merit of focusing on the trajectories of ritual activities themselves in a long and wide perspective is that it sheds light on the regional peculiarities and contingent nature of the inferred ancestor veneration, which may be overlooked in cross-cultural, ethnological arguments about the nature, role, and capacity of ancestors. It also provides a wealth of information not only to determine what types of activities took place, but also to explore the intangible symbolic significance behind those activities. As a result, this approach provides a practical solution to the justified criticism by Whitley (2002) and demonstrates how we should approach ancestor veneration and what evidence we would need in order to appropriately define it in archaeological record.