We explain new developments in classical knot theory in a 3 and 4 dimensions, i.e. we study knots in 3-space, up to isotopy as well as up to concordance. In dimension 3 we give a geometric interpretation of the Kontsevich integral (joint with Jim Conant), and in dimension 4 we introduce new concordance invariants using von Neumann signatures (joint with Tim Cochran and Kent Orr). The common geometric feature of our results is the notion of a grope cobordism.
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