**1. Introduction**

Josephson array voltage standards are the most complex superconducting integrated circuits in actual use, and represent the first and the most outstanding achievement in quantum metrology.

However some issues must still be improved for a more extended exploitation of these devices in basic metrology and in advanced precision instrumentation for applications ranging from aerospace, to defense and others.

This is mainly true when the extension of DC voltage references based on Josephson effect to AC or to the generation of programmable voltages is considered, since the circuits of these arrays are usually more complex, requiring an higher number of junctions.

In this chapter we present the most developed and the most advanced solutions studied sofar to satisfy this problem, which require to achieve the best integration and the lowest power dissipation, while preserving the basic issue of the highest noise immunity of the voltages produced by the arrays.

These characteristics should be met through the most suitable selection of some Josephson junctions parameters such as the maximum critical current density, the characteristic voltage, the junction size and the degree of hysteresis of the current voltage characteristic.

A nontrivial role is also played by the thermal stability of some of these parameters.

In order to obtain the best performances, and therefore to optimize the characteristics outlined above, we will examine different types of Josephson junctions, discussing the advantages and disadvantages of each type and showing possible research directions to choose the most suited fabrication materials and processes, considering also the possibility of using cryocoolers.

The contents of the chapter are the following:

In a first section, Josephson effect and Josephson junctions, some basic physical concepts are reviewed

In the second section, Josephson Voltage Standards: from DC to AC, the application of the quantum effect to Voltage metrology is outlined, emphasizing the specificity of the extension to AC respect to the traditional DC standard

The main focus of section 3, Current junction technologies and fabrication issues, is on the different type of Josephson junctions employed so far in circuits for programmable and AC voltage standard, with details on the link between fabrication technology and device performances.

In a last section, Special issues for next generation standard and possible solutions the present and future applications of these standards to measurement are mentioned, with main stress on the routes to overcome present limitations to these challenges
