Oskar Hagberg Approximating the rate of crossings of a function of a vector valued Gaussian process Abstract: A class of one-dimensional stationary non-Gaussian processes is the class of random processes which can be expressed as a sufficiently smooth real valued function of a vector valued differentiable, stationary Gaussian process. The crossing intensity of a fixed level is a function of the joint correlation structure of the process and its derivative, but in general no simple exact form is known. However, in 1988 Karl Breitung gave an asymptotic expression valid as the level tends to infinity and when the function is subject to certain conditions, but he didn't attempt to estimate the error. We will study how the conditions could be relaxed and how the error can be estimated as a second term in an asymptotic expansion.