This article describes the development of reliable techniques to measure the isothermal crystallization rates (ICR) under quiescent as well as under small amplitude, oscillatory shear conditions. Quiescent crystallization rates were obtained using a differential scanning calorimeter. Those under small amplitude shear were obtained using Rheometrics rheometers. It is shown how a small amount of long-chain branching in high-density polyethylene homopolymer (HDPE) dramatically influences rheological properties and enhances ICR. For these HDPEs, the rate increases with the increase in long-chain branching. The general application of isothermal crystallization studies, however, should be done with great caution. This is because the fundamentals of isothermal crystallization require that it be done on the basis of a fixed undercooling with respect to the equilibrium melting temperature. Such a temperature is ill-defined for the commercial polymers having broad molecular weight distribution (MWD). Nonetheless, a practical procedure is outlined wherein the melting curve of a previously isothermally crystallized sample is used as a substitute for judging the equilibrium melting point and in deciding the selection of a proper crystallization temperature. Even this new procedure may not be applicable for polymers having heterogeneous short-chain branching distribution.