We study how the tasks conducted on the job relate to measures of cognitive skills using data from 18 countries participating in the Programme for the International Assessment of Adult Competences (PIAAC) and from 13 countries that also participate in the International Adult Literacy Study (IALS). We document two main findings. Firstly, individual-fixed effect models suggest that low-educated workers specializing in a particular set of basic tasks -say, in numeric relative to reading or ICT tasks- obtain 10% of one standard deviation higher scores in the domain of the PIAAC assessment most related to those tasks than in the rest -say, numeracy relative to literacy or problem-solving scores. Secondly, a synthetic cohort analysis using repeated literacy assessments in IALS and PIAAC indicates that, among the low-educated, long-run increases in the reading task component of jobs correlate positively with increases in cohort-level literacy scores. The results are stronger among low-skilled workers with less working experience or females -i.e., the set of workers who have had less time to sort in the labor market. An interpretation of our findings is that tasks conducted on the job help in building human capital but are imperfect substitutes of formal schooling.