Keywords: Transit-time distribution (TTD); tracer transport; inverse problem; maximum-entropy deconvolution Abstract: I will discuss the inverse problem of inferring the Green's function for advective-diffusive transport (also known as the transit-time distribution) from tracer observations. Tracers with different boundary conditions and/or different radio-active decay rates probe different transport pathways and timescales. Using multiple tracers in combination can therefore help constrain the full transit-time distribution (TTD). I will review two inversion methodologies applicable to ocean tracer measurements, one based on a parametric model for the TTD and one based on a more flexible maximum-entropy deconvolution approach. Because the oceanographic inverse problem is grossly under-determined an important focus of this talk will be on quantifying the uncertainty associated with the inversion results.