Synthetic biology seeks to understand and derive value from biology via its re-design and synthesis using engineering principles. The workhorses of this endeavour are microbial cells, reprogrammed with synthetic DNA constructs to perform new tasks such as biosynthesis, biosensing and computation. In this talk, I will introduce my groups’ work on DNA assembly and construction of synthetic genomes before focusing on our most recent work characterising the cost synthetic biology constructs impose on the microbial cells that host them. By combining a mathematical model of protein synthesis with a novel assay for measuring the expression capacity of cells, we can assess and predict the burden different parts and constructs impose on their host and identify designs that operate efficiently at a lower cost to the cell’s economy. This foundational work adds a crucial new dimension to synthetic biology that aids the design of engineered cells more robust to mutation and with predictable growth rates desirable for use in industrial biotechnology.