This thesis offers the first in-depth analysis of the network of Puerto Rican community activism in the Lower East Side from 1964 to 2001. The community of Loisaida organized itself to fight against postwar urban deindustrialization, housing disinvestment, and gentrification, which threatened to displace an entire generation of Puerto Ricans who migrated to this New York neighborhood and tried to make it their home. Using an amalgam of unprocessed organizational archives, oral histories, ephemera, and neighborhood publications, this project recreates the history of community action in Loisaida. Focusing on key institutions and community groups that mobilized residents and built a lasting activist network, this thesis demonstrates how community groups pioneered a methodology for more sustainable community activism. These activists turned Loisaida into their laboratory, constantly experimenting with and adapting new strategies to put up a solid defense against absentee landlords, greedy developers, opportunist politicians, and an era of increased policing of urban space. The interplay of community activism, urban politics, and Puerto Rican history in Loisaida provides three crucial insights: (1) the need for grassroots organizations to adapt their activism to the changing needs of the community, (2) the creativity of urban communities to transform and design their immediate environment, and (3) the key strategies that enable activists to develop campaigns to their full potential. By uncovering these insights, this thesis raises new and challenging questions about the nature of sustained neighborhood activism at a major transitional phase in United States urban history: the shift from 1960s antipoverty programs to 1980s neoliberal policies. It shows the ingenuity and strength of activists who confronted this shift in the socio-political urban landscape by devising strategies to continue serving the residents of Loisaida. In 2017, the same community leaders who mobilized residents in the early 1960s are still marching on City Hall to demand the return of their former headquarters.