The structural and conformational properties of allylgermane have been investigated using Stark and Fourier transform microwave spectroscopies, infrared spectroscopy, and high-level quantum chemical calculations. The parent species H2C=CHCH2GeH3 was investigated by microwave spectroscopy and infrared spectroscopy, while three deuterated species, namely, H2C=CDCHGeH3, H2C CHCHDGeH3, and H2C=CHCH2GeD3, were studied only by infrared spectroscopy. The microwave spectra of the ground vibrational state as well as of the first excited state of the torsion vibration around the sp(2)-sp(3) carbon-carbon bond were assigned for the Ge-70, Ge-72, and Ge-74 isotopomers of one conformer. This rotamer has an anticlinal arrangement for the C=C-C-Ge chain of atoms. The infrared spectrum of the gas in the 500-4000 cm(-1) range has been assigned. No evidence of additional rotameric forms other than anticlinal was seen in the microwave and infrared spectra. Several different high-level ab initio and density functional theory calculations have been performed. These calculations indicate that a less stable form, having a synperiplanar conformation of the C=C-C-Ge link of atoms, may coexist with the anticlinal form. The energy differences between the synperiplanar and anticlinal forms were calculated to be 5.6-9.2 kJ/mol depending on the computational procedure. The best approximation of the equilibrium structure of the anticlinal rotamer was found in the MP2/aug-cc-pVTZ calculations. The barrier to internal rotation of the germyl group was found to be 6.561(17) kJ/mol, from measurements of the splitting of microwave transitions caused by tunneling of the germyl group through its threefold barrier.