This paper delineates mental load and stress as two related concepts t
hat originate from different theoretical frameworks. A proper distinct
ion between the two concepts is important, not only for theory buildin
g, but because it may lead also to different interpretations of experi
mental results, and, consequently, to different recommendations in app
lied situations. High workload is regarded as an important but not a c
ritical factor in the development of stress symptoms. It is quite poss
ible to work hard in difficult and complex tasks, even under unfavoura
ble conditions, without cognitive strain, psychosomatic complaints, or
adverse physiological effects. High task demands can be mct by mobili
zing extra energy through mental effort. This 'trying harder' reaction
is a normal and healthy coping strategy to adapt to situational deman
ds. In contrast, stress is regarded as a state in which the equilibriu
m between cognitive and energetical processes is disturbed by ineffect
ive energy mobilization and negative emotions. Stress typically is cha
racterized by inefficient behaviour, overreactivity, and the incapacit
y to recover from work. Stress is regarded as a state in which the phy
siological system is disorganized, which results in decreased well-bei
ng, sleeping problems, psychosomatic complaints, and increased health
risks.