Single crystal Ni-based superalloys were introduced in the early 1980s and since then have been widely used in turbine aerofoils in jet engines. The desire for weight reduction and the use of advanced metal cooling schemes tends to drive designs toward thinner airfoil walls. Creep tests on Ni-based superalloy specimens have shown greater creep strain rates and/or reduced creep rupture times for thinner specimens than is predicted by current theories. This is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single crystal superalloy sheet specimens of thicknesses 3.18mm and 0.51mm under two test conditions: 760 deg. C/758MPa and 982 deg. C/248MPa. The specimens contained initial micro-voids formed during the solidification and homogenization processes. The experiments showed that porosity evolution could play a significant role in the thickness debit effect. This motivated basic mechanics studies of porosity evolution in single crystals subject to creep loading. The evolution of porosity was studied for various values of stress triaxiality and various values of the Lode parameter. These studies will be summarized in this talk with particular attention given to the evolution of the void shape and the implication of shape changes for the near-void stress fields. Implications for the thickness debit effect will be discussed.