  We derive conditions for stable tracker solutions for both quintessence and
k-essence in a general cosmological background, H^2 \propto f(\rho). We find
that tracker solutions are possible only when \eta = d ln f /d ln \rho is
constant, aside from a few special cases, which are enumerated. Expressions for
the quintessence or k-essence equation of state are derived as a function of
\eta and the equation of state of the dominant background component.
