  A system obeying the harmonic oscillator equation of motion can be used as a
force or proper acceleration sensor. In this short review we derive analytical
expressions for the sensitivity of such sensors in a range of different
situations, considering noise of thermal and measurement origins and a
formalism for dealing with oscillators whose natural frequency $\omega_0$
jitters. A special case where the sensitivity can be improved beyond the
standard expressions and some applications with examples are also discussed.
