Applied ecology is the science of managing ecosystems for defined outcomes, such as conservation, sustainable harvest, and animal pest and weed control. Robust knowledge in science is often expressed as "principles". Principles in applied ecology have utility by assisting scientists and managers to evaluate current management and to plan future activities. Principles also have a unifying role by identifying general patterns and processes across a broad discipline. We review usage of the word principle in applied ecology by critically evaluating principles proposed previously. We identify and describe two principal uses of principles; first, a prescriptive principle defined here as a general guideline for applied ecological research and management, and second, an empirical principle defined here as a broad generalization based on replicated empirical observations and experiments. Principles proposed previously are invariably for particular applications and are not generic across applied ecology. The principles are consolidated here in a new set of 22 prescriptive and 3 empirical principles. The new principles are more comprehensive than those proposed previously and relate to all aspects of applied ecology, extending across conservation, sustainable utilization, and management of animal pests and weeds. The principles should assist applied ecologists and managers to achieve specific management objectives.