We present radio and optical observations of Holmberg I ( Ho I ) , a member of the M 81 group of galaxies ( distance \sim 3.6 Mpc ) . Ho I is a low–mass , low surface–brightness dwarf galaxy . High–resolution multi–array Very Large Array observations in the line of neutral hydrogen ( H i ) reveal a supergiant shell ( diameter 1.7 kpc ) which covers about half the optical extent of Ho I and which comprises 75 % of the total H i content ( total H i mass : 1.1 \times 10 ^ { 8 } { \cal M } _ { \sun } ) . We estimate the scaleheight of the H i layer to be 250 pc \lesssim h \lesssim 550 pc . We set a tentative upper limit to the dark matter content of \lesssim 3.1 \times 10 ^ { 8 } { \cal M } _ { \sun } . The H i data are complemented by deep , optical UBV ( RI ) _ { c } observations and narrow band H \alpha imaging obtained at the Calar Alto 2.2 m telescope . We find { \cal M } _ { HI } / L _ { B } = 1.1 { \cal M } _ { \sun } / L _ { { B } _ { \sun } } . The total visible ( stars plus gas ) mass of Ho I adds up to 2.4 \times 10 ^ { 8 } { \cal M } _ { \sun } . This leads to a total mass of \lesssim 5.5 \times 10 ^ { 8 } { \cal M } _ { \sun } and an inclination for Ho I of 10 { \arcdeg } \lesssim i \lesssim 14 { \arcdeg } . The origin of Ho I ’ s peculiar H i morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions . We estimate that the energy deposited falls in the range of 1.2 \times 10 ^ { 53 } erg \lesssim E \lesssim 2.6 \times 10 ^ { 53 } erg ( equivalent to 120–260 type II SN explosions ) . From a comparison with isochrones as well as from dynamical modeling based on the H i data we derive an age for the supergiant H i shell of \sim 80 \pm 20 Myr . The morphological center of Ho I ( i.e. , the center of the ring ) is offset by 0.75 kpc with respect to the dynamical center . Within the interior of the shell the light distribution is exponential with a rather shallow gradient and blue optical colors . Beyond a radius corresponding to an H i column density of \sim 10 ^ { 21 } cm ^ { -2 } , the putative star formation threshold , the disk becomes considerably redder and the slope for the exponential light distributions steepens . We attribute this to a uniform star formation activity in the recent past within the central 2 kpc of Ho I. Color–magnitude diagrams based on our CCD data show that the youngest stars , with ages of 15–30 Myr , are situated along the inside of the rim of the giant H i shell which is where we also find some faint H ii regions . It is speculated that these stars are the result of secondary star formation on the rim of the shell . Based on the global morphology and velocity dispersion as well as the location of the H ii regions we find evidence for ram pressure within the M 81 group . Finally , we discuss the likelihood of Ho I having lost some of its interstellar material to the intergalactic medium ( “ blow–out ” scenario ) .