To ensure quality of service (QoS), network constructors, service providers and operators need to accurately pinpoint existing and potential problems, making test and measurement equipment vital. There are a number of test tools available that address the different testing needs at various stages of the network, such as fiber commissioning. Used to reveal the total loss, optical return loss (ORL) and the fiber length, such tests can be performed either on a single fiber or on a complete network. Additionally, a closer examination of the different elements that make up the link under test may be required. Whether to characterize each component of the link, to pinpoint a potential problem with the fiber or to find a fault on your network, the use of an optical time domain reflectometer (OTDR) is inevitable -- from fiber network commissioning to troubleshooting and maintenance, an OTDR is the tool of choice. This application note explores the fundamental OTDR principles that are key to understanding the specifications of this instrument.