**Abstract: **

The canonical Boltzmann theory of transport in solid-state physics is over 60 years old. This textbook theory is sensible for most metals, where momentum-conserving electron-electron scattering is negligible. However, we have observed many diverse "strange" metals in experiment that defy this canonical theory of transport. As the electrons in such materials are strongly correlated, it is time to revisit our conventional theory of transport, properly accounting for momentum-conserving scattering. Assuming that quasiparticles are well defined, the disorder potential is smooth, and umklapp is negligible, I will present the solution to the transport problem, obtained by solving the spatially inhomogeneous Boltzmann equation. This 'mother theory' of (semi)classical transport reduces to resistor network theory, hydrodynamics, or textbook Boltzmann theory, in appropriate limits. I will describe the resistivity across the ballistic-to-hydrodynamic crossover in illustrative toy models, and give a generic answer to when electron-electron interactions enhance transport, and when they suppress transport. I will discuss experimental predictions of our formalism and compare with existing transport mysteries.