The exact conditions on valid pointer states for weak measurements are derived. It is demonstrated that weak measurements can be performed with any pointer state with vanishing probability current density. This condition is found both for weak measurements of noncommuting observables and for c-number observables. In addition, the interaction between pointer and object must be sufficiently weak. There is no restriction on the purity of the pointer state. For example, a thermal pointer state is fully valid.PACS numbers: 03.65.Ta In "orthodox" quantum mechanics, the result of a "measurement" is always one of the eigenvalues of the observable. In the last pages of his textbook on quantum mechanics [1], von Neumann provided a model of a measurement where the object under study was interacting with a measurement pointer. By assuming that the initial uncertainty of the pointer was small, von Neumann demonstrated that the pointer would display one of the eigenvalues of the object observable.Aharonov, Albert and Vaidman (AAV) considered the same experimental arrangement [2]. However, they made the opposite assumption, namely that the initial uncertainty of the pointer was large. They demonstrated that despite this, the pointer would on average show the correct expectation value of an observableĉ, although it could not distinguish separate eigenvalues. Their most interesting discovery, though, was that if a projective measurement of a second observabled was made on the object after the interaction, the average meter reading conditioned on the result of the measurement on the object would be the real part of the quantityThe authors introduced the name "weak value" for this quantity. They observed that the values of c w might lie outside the range of eigenvalues of the observableĉ. It has been contested whether the experimental arrangement of AAV qualifies as a "measurement", and whether it has any meaning to ascribe to c w a significance as a "value" of the observableĉ [3][4][5]. Despite initial scepticism, weak values have found applications in a variety of systems. A classical optical analog [6] of the original experiment proposed by AAV has been realized experimentally [7]. The polarization state of a classical radiation field can be treated as analogous to a spin-1 2 system, and the weak value of polarization may exceed the eigenvalue range [8,9]. It has also been found that weak values have applications within classical optical communication [10,11]. These examples demonstrate * Electronic address: lars.m.johansen@hibu.no that weak values have applications beyond quantum mechanics. In fact, it is not unfamiliar that the quantum formalism can be applied even to classical systems where two observables cannot be jointly measured with arbitrary accuracy. This is known e.g. in signal processing, where the Wigner distribution for time and frequency has gained popularity [12].It was recently found that "weak values" have a deeper significance when considered from the viewpoint of standard Bayesian estimation theory [13]....