Volcanic tremor occurring at the beginning of the 2018 Kīlauea eruption is characterized using both seismic and tilt data recorded at the Kīlauea summit. An automatic seismic network-based approach detects several types of tremor including (a) 0.5–1 Hz long-period tremor preceding the eruption, located at the south-southwest edge of Halema'uma'u Crater and attributed to the quasi-steady radiation from a shallow hydrothermal system and (b) two sequences of gliding tremor at the beginning of the eruption, both with locations on the edges of the crater and within it. The first sequence is attributed to two swarms of low-amplitude regularly repeating earthquakes induced by the jerky motions of a cylindrical rock piston with radius of 325 m, height of 250 m, and mass of 2.07 × 10¹¹ kg, progressively intruding 12.3 m into the shallow hydrothermal system with volume of 10⁸ m³ and depth extent of 300 m. The second sequence is attributed to a gradual evolution in the properties of a bubbly magma within an east-striking dike below Halema'uma'u Crater, impacted by repeated roof collapses. A fluid-filled crack model points to a decrease in gas volume fraction from 4.22% to 1.6 × 10⁻²% in the magma filling the dike, and a model of gas retro-diffusion within the melt suggests a two orders of magnitude decrease in bubble number density from 7 × 10⁸ m⁻³ down to 4 × 10⁶ m⁻³. Both models feature a quasi to totally degassed magma by May 26.