L-serine, L-serine precursors, L-serine derivatives and L-serine conjugates for treatment, amelioration and/or prevention of protein aggregation/tangles/plaques and diseases associated with protein aggregation/tangles/plaques. In particular, treatments and uses for L-serine, L-serine precursors, L-serine derivatives and L-serine conjugates include Alzheimer's disease (AD), Parkinson's disease, Amyotrophic Lateral Sclerosis (ALS), and Huntington disease (HD).