Interstellar dust grains catalyse chemical reactions, absorb, scatter, polarise and re-radiate starlight and constitute the building blocks for the formation of planets. Understanding this interstellar component is therefore of primary importance in many areas of astronomy & astrophysics. For example, observers need to understand how dust effects light passing through molecular clouds. Astrophysicists wish to comprehend how dust enables the collapse of clouds or how it determines the spectral behaviour of protostars, star forming regions or whole galaxies. This book gives a thorough theoretica