The remarkable political durability and economic accomplishments of China under the Chinese Communist Party (CCP) have long interested social scientists and policymakers, as the system lacks many good institutions that are considered to be essential for development and high-quality governance: Electoral institutions that regulate power transition and promote accountability are nowhere in existence except at the grassroots level. The political system has a poor reputation of being riddled with factional struggles and rampant corruption, both of which are supposed to be antithetical to political and economic development. What, then, explains the regime’s ability to endure and develop amidst these seemingly pathological features? This dissertation offers a new perspective for understanding the CCP's resilience. I argue that the regime’s remarkable performance in certain areas and its equally glaring failures in others are not inherently contradictory. Rather, they share the same origin in the unique ways in which the system operates. Departing from the existing literature, which focuses primarily on how the regime extends its longevity by improving and adapting its formal institutions to a changing political environment, my explanation traces the CCP’s durability to the enduring informal aspect of the system. With a special focus the operations of patron-client networks among the elites, I argue that, contrary to the conventional view that these informal networks are symptomatic of inefficient and corrupt patrimonial regimes, they contribute to the stability and effectiveness of the system by helping foster a sense of trust and mutual obligation among actors in a setting where intra-elite contracts are typically difficult to monitor or enforce. These particularistic relations are important vehicles for elites to organizing interests, coordinating actions, and mobilizing support; and they are often used to address some of the most pressing political and economic problems that the regime faces. More specifically, I argue that at the national level, informal networks help maintain elite cohesion by sustaining credible power-sharing arrangements among the elites. Through shared control over personnel appointments, senior political leaders can effectively lock in their power status for an extended period of time through services offered by their appointees. This informal arrangement not only allows them to closely monitor each other’s behaviors through tracking the personnel appointments, but also gives them an interest in ensuring the continuation of the regime that goes beyond their formal tenure. At the subnational level, moreover, I argue that patronage networks contribute to the regime’s economic and policy performance by supplying high-powered incentives to local agents who otherwise have a natural tendency to shirk. Contrary to the conventional wisdom that the performance incentives are generated by a formal, meritocratic selection system, I contend that the link between economic achievements and political promotions is far from institutionalized. Instead, vertical patron-client networks are the key instrument that higher-level principals use to motivate lower-level agents and generate momentum for economic development and policy implementation. Finally, I argue that although this informal institution yields measurable benefits to the regime, the benefits do not come without significant costs. Patron-client relations often involve extra-legal exchange of favors, and the need to maintain an effective and loyalty following often compels patrons to turn a blind eye on clients' illicit activities, such as corruption. This makes the regime’s anticorruption enforcement a highly selective undertaking and a non-credible deterrent against official venality. The system's inability to eradicate corruption, therefore, is not a simple result of policy or strategic failures, but rather a direct price that the regime has to pay for its effectiveness. I provide empirical support to our argument by investigating how informal networks shape several critical aspects of Chinese politics, including (1) political institutionalization, (2) economic development, (3) policy implementation, and (4) anticorruption enforcement. The main quantitative evidence is drawn from an original biographical database of over 4,000 politicians at city, provincial, and national levels since early 1990s. New automated algorithms are developed to identify patron-client ties by linking lower-level officials with political superiors who oversaw their major promotions in the past. To supplement the quantitative analyses, I also draw evidence from in-depth interviews with serving and retired officials in both central and local governments and additional historical and archival materials during my fieldwork in China.