Abstract: Josephson weak links provide a means for investigating the properties of quantum systems as well as being very useful components in the design of practical devices such as the dc SQUID. This dissertation summarizes the current progress in developing superfluid helium weak links along with the study of their associated Josephson effects, including the theoretical concepts and the experimental realizations in a superfluid 3He system. This research has culminated in the development and experimental success of a superfluid 3He dc SQUID and the first observation of quantum interference in a liquid. By using a hydromechanical resonator, we find that an array of orifices separating two volumes of superfluid 3He acts as a single weak link, displaying not only the known Josephson effects, including the superfluid Fiske effect and the long sought after superfluid Shapiro effect, but some entirely new phenomenon associated with the superfluid 3He order parameter. We report the discovery of π-states, bi-stability and new dissipation mechanisms in a superfluid 3He weak link array. The quantitative measurements made using a single superfluid 3He weak link array are used in the development of a superfluid 3He dc SQUID which incorporates two superfluid weak link arrays. We test the operation of the superfluid dc SQUID by using the Earth as a rotating platform which can induce additional quantum phase differences within the superfluid. The dc SQUID is shown to act as a single weak link which exhibits Josephson effects as a whole and the critical current for the device is modulated by the addition of circulation flux in accordance with theoretical predictions. This allows the device to be used as a rotation sensor or gyroscope. Through the development of the superfluid 3He dc SQUID, it has been possible to observe multiple possible order parameter configurations or states, at the same temperature and ambient pressure, for the superfluid 3He weak link arrays which make up the device. We also find that this device can sustain a half-quantum of trapped circulation. This could be an entirely new Josephson effect due to the p-wave nature of the order parameter, where the entire current-phase relation for one of the weak link arrays has been shifted by π.