The literature on convergence in per-capita income across countries has not converged on a common concept of convergence. It may be within a country towards its own steady state or between countries. Between country convergence may be absolute convergence to the same steady state; conditional convergence to country specific steady states, functions of observed variables; or club convergence to different steady states. It may be measured by beta convergence; sigma convergence; or the presence of a common trend. This paper surveys the econometric issues involved in estimating the rate of convergence; testing for convergence; and specifying the unobserved steady state. The survey suggests that rather than there being different ways to measure a single concept, convergence, the different measures are measuring different things.