This technical report provides system design methodology guidance to users and suppliers of fibre optic transmission systems for telecommunications and broadband video distribution applications. The function of the fibre optic systems considered in this technical report is to interconnect signals between defined digital and analogue interfaces via a fibre optic link. In general, fibre optic systems are built-up from basic fibre optic systems (BFOS). As a result, it is necessary to consider only the BFOS, which transmits a single electrical signal between two points without intermediate regeneration. Thus, these guidelines are applicable to almost all design cases. As an initial step, only laser systems using direct detecting technologies in the 1 310 and/or 1 550 nm regions over B1 and B2 types of unshifted and dispersion shifted single-mode fibres are considered. For systems using longitudinal multi-mode lasers above approximately 0,5 Gb/s, the calculated dispersion limited lengths may be conservatively low. Bidirectional systems, in which both directions of transmission use a common fibre, and unidirectional systems are covered in this technical report. In the clauses which follow, the transmission design parameters are specified by their worstcase values. It is the option of the user to require that suppliers guarantee the worst-case values over the lifetime of their respective system components. Some users or system designers may desire to perform analysis and design calculations via a statistical approach and to request statistical values of the parameters. The statistical approach is covered in annex A. The determination of the worst-case dispersion coefficient is covered in annex B. Some examples are presented to clarify concepts and to illustrate order of magnitude. The numbers presented in these examples are illustrative only and should not be considered as requirements for system design or performance.