The low frequency (<EQ 10 MHz) electrical impedance of a volume of tissue is sensitive to its temperature and its response to heating and other stresses. Major tissue changes, such as those accompanying higher hyperthermic temperatures or prolonged ischemia, and not necessarily reversible unless detected in time to alleviate the stress. Thus, it is imperative to assess the temperature and/or tissue changes in real-time if adequate monitoring of thermal treatments is to be accomplished. To this end, we focus on the use of electrical impedance measurements of a volume of tissue at temperatures in the hyperthermia region (<EQ 47 degree(s)C) where tissue responses occur at a rate which is controllable. First, using well controlled freshly excised tissue data, we examine the prototypical impedance changes associated with the early and later stages of necrosis within a tissue subjected to heating and ischemia. Then, impedance measurements made non-invasively, in vivo, in HT29 tumors are used to demonstrate the differences caused by different thermal treatments and from differences in the ischemic condition of the tissues. The electrical impedance signature of the tumors were indicative of certain cellular-level changes occurring within the tumors. The histological findings corroborate the ability of the electrical impedance to report these cellular changes. The changes are consistent with the cells proceeding along the path of necrosis. Initial cell swelling appears to be largely due to ischemia, and the cell lysing and tumor response to the defined period of heating.