  Because of their unique designs, the Pioneer 10 and 11 spacecraft have
provided the cleanest Doppler, deep-space navigation data. Analysis of this
data can be interpreted as showing an anomalous acceleration of these craft
directed towards the Sun of $a_P \sim 8 \times 10^{-8} {\rm cm/s}^2$. The
background of this discovery and the significance of the result are discussed.
