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Superconductor-Insulator-Superconductor (SIS) mixers are now used regularly in sub- millimetre astronomical receivers. They have already achieved sensitivity approaching the quantum limit at frequencies below the superconducting gap of niobium (~680 GHz). Above that, the mixer performance is compromised by losses, unless materials with higher superconducting gap are employed in conjunction with the niobium tunnel junction. In this thesis, we present the development of 700 GHz niobium SIS mixers, employing a unilateral finline taper on a thin Silicon-On-Insulator (SOI) substrate. These mixers are broadband with full on-chip planar circuit integration, and require only a very simple mixer block. They were designed using rigorous 3-D electromagnetic simulator (HFSS), in conjunction with a quantum mixing software package (SuperMix), and have demonstrated good performance with the best noise temperature measured at 143 K. Our mixer devices were fed by multiple flare angle smooth-walled horns, which are easy to fabricated, yet retain the high performance of corrugated horns. The radiation patterns measured from 600–740 GHz have shown good beam circularity, low sidelobe and cross-polarization levels. In this thesis, we also present SIS mixer designs with balanced and sideband separ- ating capability. These mixers employ back-to-back finline tapers, so that the RF and local oscillator (LO) signals can be injected separately without a beam splitter. We have fabricated and tested the performance of the balanced mixers, and analysed their performance thoroughly. We have also investigated a new method of generating LO signals by beating the tones of two infrared lasers. Using the current 16-pixel 350 GHz SIS receiver, HARP-B, we have observed the 12CO J=3→2 emission lines from two nearby galaxies. An important result we found is that the 12CO J=3→2 correlates strongly with the 8 μm Polycyclic Aromatic Hydrocarbon emission.