1.  Introduction

2.  Porosity related to star formation:  an analytical approach

    2.1  The volume of hot gas generated by star formation:  steady state
    2.2  Non-steady star formation
    2.3  Effect of cooling and finite scale of ISM
    2.4  Calculation of galactic porosity

3.  Consequences for star formation efficiency and escape of ionising radiation

    3.1  Star-formation efficiency
    3.2  Implications for the escape of ionising radiation

4.  Applications to star-forming systems

    4.1  Compact systems
    4.2  Milky Way
    4.3  Lyman Break Galaxies
    4.4  Starbursts at low z

5.  Conclusions