The external linkage system concerned with that firms search for and acquire technological knowledge and information is focused on. Using product/profit cycle theory as a starting point, an evolutionary nature of the system along with the development of a sector’s technology is asserted and the system in an emergent sector is compared with that in a mature sector. An empirical survey of electronics and ICT firms and automotive and related engineering firms in West Midlands of UK provides evidence supporting the system’s evolutionary nature. Reflecting on the growth period of these sectors, electronics and ICT firms are more likely to use external sources of technical knowledge, have a diversified set of sources, and seek for tacit knowledge than automotive and related engineering firms. However, it is found that their differences in the relationship with external sources are subtle. Firms in both the sectors seek and keep up long-term, stable relationships with their important external sources of technical knowledge.