Over the past two decades, the PPP user model has constantly been evolving. With each iteration, improvements were made primarily in regards to accuracy and most notably convergence. The standard practice in conventional PPP has been to linearly combine two pseudoranges and two carrier-phases to produce ionosphere-free linear pseudoranges and carrier-phase combinations, which eliminates the first order ionosphere delay. Originally, the ionosphere delay was considered a nuisance parameter within the positioning community. As a result, the ionosphere-free linear combination was favoured in contrast to the estimation of the slant ionosphere delay. Nowadays, the PPP model permits multi-frequency, multi-constellation, slant ionosphere estimation and ambiguity resolution. Presented in each of the following sections is a review of the steps the PPP user model underwent within its evolution.