Keywords: Transit-time distribution (TTD); tracer transport rates and pathways Abstract: I will explain the concept of transit-time distributions (TTDs) to diagnose passive tracer transport in geophysical fluids, like the Earth's ocean and atmosphere. The TTD is directly related to the Green's function to the advection/diffusion equation for the concentration of a dynamically-passive trace substance. Diagnosing and interpreting the TTD, rather than the passive tracer field itself, focuses attention on the advective/diffusive transport properties of the underlying flow, and removes the influence of the tracer sources and sinks. The TTD therefore quantifies fundamental transport rate and pathway information about the flow. Various basic mathematical constraints on the TTD function exist, which may illuminate traditional diagnostics of tracer stirring and mixing. Applications of the TTD approach are presented in ocean circulation models, and using ocean tracer measurements.