  Pulsar Timing Arrays use a set of millisecond pulsars in an attempt to
directly detect nanohertz gravitational waves. For this purpose, high precision
timing of the pulsars is essential and ultimately a precision of the order of
~100 ns is required. Propagation effects in the interstellar medium cause the
radio emission from a pulsar to be dispersed and scattered, introducing time
variable delays of the pulses on their way to Earth. If these delays are not
properly corrected for, they may cause significant errors in the timing
analysis of a pulsar. These proceedings will review the effects of the
interstellar medium on pulse arrival times and present some of the techniques
used to mitigate the associated time delays from the pulsar signal. Correcting
for these delays is essential to providing a higher timing precision and hence
to increasing the array's sensitivity to gravitational waves.
