This paper is based on a report of the same title, prepared by Working Group K3 of the Substation Protection Subcommittee of the Power System Relaying Committee of the Power Engineering Society of the IEEE. The paper begins with background information on the causes, measurement techniques, and consequences of overheating in mineral-oil-immersed power transformers. Then techniques for adaptive transformer thermal overload protection based on the real-time solution of the transient heating equations are presented. An adaptive overcurrent protection implementation, and the related functional requirements, are discussed. The equations of Clause 7 of the 1995 IEEE Guide for Loading Mineral-Oil-Immersed Transformers are used in the analysis, with a slight modification to allow for continuously varying ambient temperature.