This study diagnosed the understanding about energy and biological-context energy concepts held by 90 first-year South African university biology students. In particular, students' explanations of energy in a biological context, how energy is involved in different biological situations and whether energy is present and what types of energy are involved in diagrams depicting biological phenomena were investigated. The pencil-and-paper diagnostic test, specifically designed for this study, was used to elicit students' understanding using test items involving biological phenomena. The results showed that many students had problems in understanding energy and energy-related concepts in the following areas: First, the majority of the students provided "definitions" of energy rather than the "explanations" they were asked to provide, and the definition could have been rote-learned. Second, although nearly all students knew the energy conservation principle (energy cannot be created or destroyed), many of them were unable to apply this concept to biological contexts. Third, many students erroneously claimed that the energy for metabolism and life processes is made available during photosynthesis in plants, during digestion in animals or that this energy comes "directly" from the sun. Fourth, about two thirds of the students erroneously indicated that there is no energy involved/present in inanimate objects such as a statue. The implications for the teaching and learning of energy and its related concepts and recommendations for further research are discussed.