Mechanical engineers often are forced to be concerned with cavitation in flows near bodies or within machinery. Viscous effects on cavitation introduce difficulties in design, because these effects often are significantly different for model and full-scale conditions. Therefore, a theoretical analysis would be desirable in numerical prediction of full-scale performances and in interpretation of experimental data. Though cavitation is a very complex nonlinear phenomenon, the numerical analysis of which is difficult even approaching an ideal fluid model, recently certain successes were obtained in the examination of viscous effects on sheet cavitation and vortex cavitation. A part of the theoretical results concerning viscous effects on cavitation was provided by matches of well-known solutions for boundary layers over non-cavitating bodies and solutions for cavities in an ideal fluid. The review of theoretical methods and consideration of the best way to proceed with future research is the objective of this review article, which includes 92 references.