Measurement of subjective work load has emerged as a central topic of interest in the study of physical work and effort. From the practical point of view, subjective estimates of physical effort can contribute greatly to the assessment of work loads in physical activities and to the design of manual handling tasks. The rationale of this approach is that subjective estimates reflect the individual perception of the amount of physical and motivational resources that the subjects invest to meet the demand imposed by the physical task. The present paper provides an illustration of some of the techniques used in measuring the perception of physical effort and reviews and discusses the main results obtained with these techniques in the assessment of manual materials handling.