Methane production by anaerobic digestion (AD) of macroalgae (seaweed) is a promising algal bioenergy option. Work presented here is primarily based on the AD of Laminaria hyperborea using batch and continuously stirred tank reactors. Extrapolation of data from batch studies to long term continuous reactors was unreliable. A conservative organic loading rate (OLR) of 1 g L−1 d−1 was used due to difficulties experienced in achieving steady state performance at an OLR of 1.5 g L−1 d−1. Biogas composition and methane yields (60–70%) were near to values expected from terrestrial feedstocks. Biomass washout, as imposed by the dilution rate (i.e., hydraulic residence), had considerable bearing on the biogas generation profile, particularly at >3 hydraulic residences. Inhibition of methanogen growth was linked to nutrient deficiency and potentially antimicrobial compounds associated with the feedstock. Anaerobic digestion of L. hyperborea proved feasible over extended operational periods.