Industrial production of β-lactam antibiotics by the filamentous fungus Penicillium chrysogenum is based on successive classical strain improvement cycles. This review summarizes our current knowledge on the results of this classical strain improvement process, and discusses avenues to improve β-lactam biosynthesis and to exploit P. chrysogenum as an industrial host for the production of other antibiotics and peptide products. Genomic and transcriptional analysis of strain lineages has led to the identification of several important alterations in high-yielding strains, including the amplification of the penicillin biosynthetic gene cluster, elevated transcription of genes involved in biosynthesis of penicillin and amino acid precursors, and genes encoding microbody proliferation factors. In recent years, successful metabolic engineering and synthetic biology approaches have resulted in the redirection of the penicillin pathway towards the production of cephalosporins. This sets a new direction in industrial antibiotics productions towards more sustainable methods for the fermentative production of unnatural antibiotics and related compounds.