. Based o~ simple principles, s l~ectrop hotoI?ctry nevertheless demands a lot of precautIOns to aVOi d errors. The followmg properties of spectrophotometers will be discussed together with methods to test them:~pectra~ properties-wavelength accuracy, bandwidth, stray light; photometri c linearity; mteractlOns between sample a~d instrument-multiple reflections, polarization, divergence, sample wedge, sample tIlt, optical path length (refr active index), interferences .Calibratio~ of master instruments is feasiJ:>le only by complicated procedures. With such a master mstrument standards may be calIbrated which greatly simplify performance checks of mst:u:nen~s used for practical work. For testing high quality spcctrophotometcrs the u~e of emiSSIOn lmes and nearly neutral absorbing solid filters as standards seems to be supenor, for some kinds of routine instruments the use of absorption bands and liquid filters may be necessary.