ABSTRACT: It is usually assumed that a balanced homodyne detector measures a variable called the quadrature phase amplitude of a signal field. The author examines this assumption by obtaining analytic expressions for the probability statistics for the output of such a detector for a single mode signal with both signal and local oscillator treated quantum mechanically. He investigates the conditions under which the statistics of the quadrature phase are reproduced in the actual output of the detector. It is shown that the most obvious condition-that the number of quanta in the local oscillator be much larger than the number in the signal-is not sufficient to ensure that a balanced homodyne detector acts like an ideal detector of quadrature phases. Furthermore, explicit conditions are obtained that are necessary and sufficient to reproduce the interference features in the homodyne statistics of a superposition of coherent states.