The matter formed in central heavy-ion collisions at a few GeV per nucleon is commonly understood as resonance matter, a gas of nucleons and excited baryonic states with a substantial contribution from mesonic, mostly pionic excitations. Yet, in the initial phase of the reaction the system is compressed to beyond nuclear ground state density and hence substantial modifications of the hadron properties are expected to occur. It is conjectured that at high enough densities hadronic degrees of freedom would finally disappear and a chirally restored phase of quarks would emerge. In this contribution we present key results on in-medium properties of hadrons obtained by the High Acceptance DiElectron Spectrometer. The spectral distribution of virtual photons emitted from the collision zone of A+A collisions indicates strong medium effects beyond those resulting from a pure superposition of individual N+N collisions. This observable, as well as the measured hadron abundances in the final state, show features of a thermalized fireball. Baryon-driven medium effects influence significantly the $\rho$ meson in-medium spectral function and are considered essential in describing the low-mass dilepton spectra. While the measured abundance of all reconstructed particles are well described assuming thermalization, the double strange cascade $\Xi^{-}$(1321) production in A+A and p+A collisions exhibits a sizeable enhancement above predictions of statistical hadronization and transport model calculations. A deeper understanding of the microscopic properties of resonance matter requires systematic investigation of baryonic decays and these are studied in HADES making use of pion beams. This experimental program will be continued in the coming years with the upgraded HADES detector.