The Normal Operations Safety Survey (NOSS) is an observational methodology to collect safety data during normal Air Traffic Control (ATC) operations. It aims to inform organizations about safety matters by using trained ATC staff to take a structured look at everyday operations. By monitoring normal operations through the use of direct over-the-shoulder observations, it is believed that safety deficiencies can be identified in a proactive manner prior to the occurrence of accidents or incidents. NOSS was developed as a collaborative effort between the International Civil Aviation Organization, ATC providers, controller representatives, government regulators, and academics to fill a gap in available ATC safety information. System designers consider three basic assumptions: the technology needed to achieve the system production goals, the training necessary for people to operate the technology, and the regulations that dictate system behavior. These assumptions represent the expected performance. When systems are deployed, however, particularly in realms as complex as ATC, they do not perform quite as designed. NOSS aims to capture the operational drift that invariably occurs upon system deployment. NOSS captures how the ATC system operates in reality, as opposed to how it was intended to operate. NOSS is premised on the Threat and Error Management (TEM) framework. TEM frames human performance in complex and dynamic settings from an operational perspective by simultaneously focusing on the environment and how operators respond to that environment. TEM posits that threats and errors are a part of everyday operations in ATC and must be managed in order to maintain safety margins. This dissertation describes NOSS and its contributions to ATC safety management systems. It addresses the validity and reliability of NOSS data and presents case studies from field trials conducted by a number of ATC providers. / text