Composting is nowadays a general treatment method for municipal solid waste. Compostable household waste contains, together with vegetable material, varying amounts of papers and boards. In the European Union composting is regarded as one recycling method for packages and this will probably favour compostable packages, like papers and boards, in the future. Paper is made up of lignocellulose and it may contain up to 20% of lignin. Efficient degradation of papers in composting plants means that biodegradation of lignin is also needed. However, very little is known about lignin degradation by mixed microbial compost populations, although lignin degradation by white-rot fungi has been extensively studied in recent years. Organic material is converted to carbon dioxide, humus, and heat by compost microorganisms. It is assumed that humus is formed mainly from lignin. Thus, lignin is not totally mineralized during composting. The elevated temperatures found during the thermophilic phase are essential for rapid degradation of lignocellulose. Complex organic compounds like lignin are mainly degraded by thermophilic microfungi and actinomycetes. The optimum temperature for thermophilic fungi is 40–50°C which is also the optimum temperature for lignin degradation in compost.