  Conventional CCD detectors have two major disadvantages: they are slow to
read out and they suffer from read noise. These problems combine to make
high-speed spectroscopy of faint targets the most demanding of astronomical
observations. It is possible to overcome these weaknesses by using
electron-multiplying CCDs (EMCCDs). EMCCDs are conventional frame-transfer
CCDs, but with an extended serial register containing high-voltage electrodes.
An avalanche of secondary electrons is produced as the photon-generated
electrons are clocked through this register, resulting in signal amplification
that renders the read noise negligible. Using a combination of laboratory
measurements with the QUCAM2 EMCCD camera and Monte Carlo modelling, we show
that it is possible to significantly increase the signal-to-noise ratio of an
observation by using an EMCCD, but only if it is optimised and utilised
correctly. We also show that even greater gains are possible through the use of
photon counting. We present a recipe for astronomers to follow when setting up
a typical EMCCD observation which ensures that maximum signal-to-noise ratio is
obtained. We also discuss the benefits that EMCCDs would bring if used with the
next generation of extremely large telescopes. Although we mainly consider the
spectroscopic use of EMCCDs, our conclusions are equally applicable to imaging.
